Tweak note about 'IDs' being keyword to also mention 'codes' (#157)

README.md

@@ -516,11 +516,17 @@ of a breaking change with these two widely used fields in Beats.
 Any future field that will be indexed for full text search in ECS will however
 follow the multi-field convention where `text` indexing is nested in the multi-field.
 
-### IDs are keywords not integers
+### IDs and most codes are keywords, not integers
 
-Despite the fact that IDs are often integers in various systems, this is not
-always the case. Since we want to make it possible to map as many data sources
-to ECS as possible, we default to using the `keyword` type for IDs.
+Despite the fact that IDs and codes (e.g. error codes) are often integers,
+this is not always the case.
+Since we want to make it possible to map as many systems and data sources
+to ECS as possible, we default to using the `keyword` type for IDs and codes.
+
+Some specific kinds of codes are always integers, like HTTP status codes.
+If those have a specific corresponding specific field (as HTTP status does),
+its type can safely be an integer type.
+But generic field like `error.code` cannot have this guarantee, and are therefore `keyword`.
 
 # <a name="about-ecs"></a>FAQ
 

docs/implementing.md

@@ -71,8 +71,14 @@ of a breaking change with these two widely used fields in Beats.
 Any future field that will be indexed for full text search in ECS will however
 follow the multi-field convention where `text` indexing is nested in the multi-field.
 
-### IDs are keywords not integers
+### IDs and most codes are keywords, not integers
 
-Despite the fact that IDs are often integers in various systems, this is not
-always the case. Since we want to make it possible to map as many data sources
-to ECS as possible, we default to using the `keyword` type for IDs.
+Despite the fact that IDs and codes (e.g. error codes) are often integers,
+this is not always the case.
+Since we want to make it possible to map as many systems and data sources
+to ECS as possible, we default to using the `keyword` type for IDs and codes.
+
+Some specific kinds of codes are always integers, like HTTP status codes.
+If those have a specific corresponding specific field (as HTTP status does),
+its type can safely be an integer type.
+But generic field like `error.code` cannot have this guarantee, and are therefore `keyword`.