diff --git a/README.md b/README.md
index ea5cec0..6c684c3 100644
--- a/README.md
+++ b/README.md
@@ -117,7 +117,7 @@ for instance the [ORCID Identifier](./entities/ORCID_Identifier.md).
 * [ROR ID Type](./datatypes/ROR_ID.md)
 * [URI Type](./datatypes/URI.md)
 * [DOI Type](./datatypes/DOI.md)
-* [Fundref ID Type](./datatypes/Fundref_ID.md)
+* [Fundref ID Type](./datatypes/FundRef_ID.md)
 * [ISNI ID Type](./datatypes/ISNI_ID.md)
 * [Wikidata ID Type](./datatypes/Wikidata_ID.md)
 * [Person Name](./datatypes/Person_Name.md)
diff --git a/datatypes/Fundref_ID.md b/datatypes/FundRef_ID.md
similarity index 94%
rename from datatypes/Fundref_ID.md
rename to datatypes/FundRef_ID.md
index 058ae52..7437ffd 100644
--- a/datatypes/Fundref_ID.md
+++ b/datatypes/FundRef_ID.md
@@ -1,4 +1,4 @@
-# The Fundref ID datatype
+# The FundRef ID datatype
 
 The datatype that represents [Funder registry](https://www.crossref.org/services/funder-registry/) entries.
 
diff --git a/datatypes/ORCID_iD.md b/datatypes/ORCID_iD.md
index 88a58da..e138fa4 100644
--- a/datatypes/ORCID_iD.md
+++ b/datatypes/ORCID_iD.md
@@ -1,6 +1,6 @@
 # ORCID iD Type
 
-The datatype that represents the [ORCID](https://orcid.org/) identifier.
+The datatype that represents the [ORCID identifier](../entities/ORCID_Identifier.md).
 
 It is based on the [String](../datatypes/String.md) datatype with the following restriction:
 - the string must match one of the following regular expressions:<sup>[1](#fn1)</sup> 
diff --git a/datatypes/Scopus_Author_ID.md b/datatypes/Scopus_Author_ID.md
new file mode 100644
index 0000000..3a1d180
--- /dev/null
+++ b/datatypes/Scopus_Author_ID.md
@@ -0,0 +1,8 @@
+# Scopus Author ID Type
+
+The datatype that represents the [Scopus Author identifier](../entities/Scopus_Author_Identifier.md).
+
+It is based on the [Decimal](../datatypes/Decimal.md) datatype with the following restriction:
+- it is a positive integer usually with 10 or 11 digits
+
+
diff --git a/entities/Agent.md b/entities/Agent.md
index 5c4ef57..66c848d 100644
--- a/entities/Agent.md
+++ b/entities/Agent.md
@@ -14,10 +14,6 @@ description: [Multilingual String](../datatypes/Multilingual_String.md)
 
 keywords: List<[Multilingual String](../datatypes/Multilingual_String.md)>
 
-ISNI ID : [ISNI ID Type](../datatypes/ISNI_ID.md)
-
-Wikidata ID : [Wikidata ID Type](../datatypes/Wikidata_ID.md)
-
 lifetime : [Date Range](../datatypes/Date_Range.md)
 
 ## Relationships
diff --git a/entities/FundRef_Identifier.md b/entities/FundRef_Identifier.md
new file mode 100644
index 0000000..dedf047
--- /dev/null
+++ b/entities/FundRef_Identifier.md
@@ -0,0 +1,22 @@
+# FundRef Identifier
+
+## Definition
+A FundRef<sup>[1](#fn1)</sup> identifier assigned to an [Organisation unit](../entities/Organisation_Unit.md).
+
+## Specialization of
+[Agent Identifier](../entities/Agent_Identifier.md)
+
+## Attributes
+Besides those of [Agent Identifier](../entities/Agent_Identifier.md):
+
+FundRef ID: [FundFef ID Type](../datatypes/FundRef_ID.md) for organizations that also fund research
+
+## Relationships
+None besides those of [Agent Identifier](../entities/Agent_Identifier.md).
+
+## Constraints
+A FundRef Identifier can only [be assigned](../entities/Agent_Identifier.md#user-content-rel__is-assigned-to) to an [Organisation](../entities/Organisation_Unit.md) that also fund research, not other subtypes of [Agent](../entities/Agent.md).
+
+---
+## References
+<a name="fn1">\[1\]</a> *Crossref funder ID*. Wikidata property. Available from https://www.wikidata.org/wiki/Property:P3153
diff --git a/entities/ISNI_Identifier.md b/entities/ISNI_Identifier.md
new file mode 100644
index 0000000..dbd6be0
--- /dev/null
+++ b/entities/ISNI_Identifier.md
@@ -0,0 +1,19 @@
+# ISNI Identifier
+
+## Definition
+An ISNI<sup>[1](#fn1)</sup> identifier assigned to an [Agent](../entities/Agent.md).
+
+## Specialization of
+[Agent Identifier](../entities/Agent_Identifier.md)
+
+## Attributes
+Besides those of [Agent Identifier](../entities/Agent_Identifier.md):
+
+ISNI ID : [ISNI ID Type](../datatypes/ISNI_ID.md)
+
+## Relationships
+None besides those of [Agent Identifier](../entities/Agent_Identifier.md).
+
+---
+## References
+<a name="fn1">\[1\]</a> What is ISNI? https://isni.org/page/what-is-isni/ 
diff --git a/entities/ORCID_Identifier.md b/entities/ORCID_Identifier.md
index 14c9b9f..d179fe7 100644
--- a/entities/ORCID_Identifier.md
+++ b/entities/ORCID_Identifier.md
@@ -9,7 +9,7 @@ An ORCID<sup>[1](#fn1)</sup> identifier assigned to a [Person](../entities/Perso
 ## Attributes
 Besides those of [Agent Identifier](../entities/Agent_Identifier.md):
 
-<a name="DOI">ORCID ID : [ORCID iD Type](../datatypes/ORCID_iD.md)</a>
+ORCID ID : [ORCID iD Type](../datatypes/ORCID_iD.md)
 
 ## Relationships
 None besides those of [Agent Identifier](../entities/Agent_Identifier.md).
diff --git a/entities/Organisation_Unit.md b/entities/Organisation_Unit.md
index f1cd01a..fc541e4 100644
--- a/entities/Organisation_Unit.md
+++ b/entities/Organisation_Unit.md
@@ -24,10 +24,6 @@ acronym: [String](../datatypes/String.md)
 
 address: [Postal Address](../datatypes/Postal_Address.md)
 
-ROR ID: [ROR ID Type](../datatypes/ROR_ID.md)
-
-Fundref ID: [Fundref ID Type](../datatypes/Fundref_ID.md) for organizations that also fund research
-
 web site URL: [URI](../datatypes/URI.md)
 
 email: [URI](../datatypes/URI.md)
diff --git a/entities/Person.md b/entities/Person.md
index 98741cc..4f9283e 100644
--- a/entities/Person.md
+++ b/entities/Person.md
@@ -19,11 +19,9 @@ One Person typically has many of these relationships.
 ## Attributes
 name: [Person Name](../datatypes/Person_Name.md)
 
-FIXME - move to ORCID_Identifier
-ORCID iD: [ORCID iD](../datatypes/ORCID_iD.md)
+ORCID iD: [ORCID Identifier](../entities/ORCID_Identifier.md)
 
-FIXME - move to Scopus_Author_ID_Identifier
-Scopus Author ID: [Scopus Author ID](../datatypes/Scopus_Author_ID.md)
+Scopus Author ID: [Scopus Author Identifier](../entities/Scopus_Author_Identifier.md)
 
 ## Relationships
 
diff --git a/entities/ROR_Identifier.md b/entities/ROR_Identifier.md
new file mode 100644
index 0000000..5ff595b
--- /dev/null
+++ b/entities/ROR_Identifier.md
@@ -0,0 +1,22 @@
+# ROR Identifier
+
+## Definition
+A ROR<sup>[1](#fn1)</sup> identifier assigned to an [Organisation unit](../entities/Organisation_Unit.md).
+
+## Specialization of
+[Agent Identifier](../entities/Agent_Identifier.md)
+
+## Attributes
+Besides those of [Agent Identifier](../entities/Agent_Identifier.md):
+
+ROR ID: [ROR ID Type](../datatypes/ROR_ID.md)
+
+## Relationships
+None besides those of [Agent Identifier](../entities/Agent_Identifier.md).
+
+## Constraints
+A ROR Identifier can only [be assigned](../entities/Agent_Identifier.md#user-content-rel__is-assigned-to) to an [Organisation unit](../entities/Organisation_Unit.md), not other subtypes of [Agent](../entities/Agent.md).
+
+---
+## References
+<a name="fn1">\[1\]</a> *Facts*. ROR. Available from https://ror.org/facts/
diff --git a/entities/Scopus_Author_Identifier.md b/entities/Scopus_Author_Identifier.md
new file mode 100644
index 0000000..90eb7df
--- /dev/null
+++ b/entities/Scopus_Author_Identifier.md
@@ -0,0 +1,22 @@
+# Scopus Author Identifier
+
+## Definition
+A Scopus Author <sup>[1](#fn1)</sup> identifier assigned to a [Person](../entities/Person.md).
+
+## Specialization of
+[Agent Identifier](../entities/Agent_Identifier.md)
+
+## Attributes
+Besides those of [Agent Identifier](../entities/Agent_Identifier.md):
+
+Scopus Author Identifier : [Scopus Author ID Type](../datatypes/Scopus_Author_ID.md)
+
+## Relationships
+None besides those of [Agent Identifier](../entities/Agent_Identifier.md).
+
+## Constraints
+A Scopus Author Identifier can only [be assigned](../entities/Agent_Identifier.md#user-content-rel__is-assigned-to) to a [Person](../entities/Person.md), not other subtypes of [Agent](../entities/Agent.md).
+
+---
+## References
+<a name="fn1">\[1\]</a> What is the Scopus Author Identifier? https://service.elsevier.com/app/answers/detail/a_id/11212/supporthub/scopus/ 
diff --git a/entities/Wikidata_Identifier.md b/entities/Wikidata_Identifier.md
new file mode 100644
index 0000000..5fad1fa
--- /dev/null
+++ b/entities/Wikidata_Identifier.md
@@ -0,0 +1,19 @@
+# Wikidata Identifier
+
+## Definition
+A Wikidata<sup>[1](#fn1)</sup> identifier assigned to an [Agent](../entities/Agent.md).
+
+## Specialization of
+[Agent Identifier](../entities/Agent_Identifier.md)
+
+## Attributes
+Besides those of [Agent Identifier](../entities/Agent_Identifier.md):
+
+Wikidata ID : [Wikidata ID Type](../datatypes/Wikidata_ID.md)
+
+## Relationships
+None besides those of [Agent Identifier](../entities/Agent_Identifier.md).
+
+---
+## References
+<a name="fn1">\[1\]</a> Wikidata:Identifiers, https://www.wikidata.org/wiki/Wikidata:Identifiers