We only have one location in CMISCore.xsd that uses the "abstract" attribute:

<xs:complexType name="cmisTypeDefinitionType" abstract="false">

This seems not to make too much sense. Either we ignore the existence of the abstract capability completely and remove this attribute here. Or we use it consistently within the schema which raises the followinig questions:
1) Why is it set to false, cmisTypeDefinitionType should be an abstract type? This is confusing.
2) We have other types in the schema that are abstract too, e.g. cmisPropertyDefinitionType that do not use abstractl.

The default is probably false anyway so removing it would be easiest solution. Making use of abstract=true would allow better validation.