disallow use of both instantiationPhysical and instantiationDigital

[kvanmalssen]

Currently the schema allows for both instantiationPhysical and instantiationDigital to be used for the same instantiation. If the elements and their usage stays as is (depending on the outcome of #67), the schema should be modified so that only one of the two is allowed, using xsd:choice. For example:

<xsd:choice>
    <xsd:element name="instantiationPhysical" type="sourceVersionStringType"/>
    <xsd:element name="instantiationDigital" type="sourceVersionStringType"/>
</xsd:choice>

[jolene2323]

Agreed! This has never made sense to me. We use instantiationPhysical for our physical tape instantiations and instantiationDigital for audiofiles on servers.

[asteino]

I could see where it would be useful to allow both and disallow both. In addition to disallowing the use of both instantiationPhysical and instantiationDigital, a reference element to another instantiation (either physical or digital) could be added (if there isn't one already).