I am having great difficulty understanding the Note at the end of 2.3.1:

<quote>
In addition to the errors defined in this specification, an implementation may raise a dynamic error for a reason beyond the scope of this specification. For example, limitations may exist on the maximum numbers or sizes of various objects. An error must be raised if such a limitation is exceeded [err:XPDY0130].

Note:

Any limits on primitives defined by the XML and XSD specifications that differ from what these specifications state are implementation-defined, and must be documented. See 5.3 Data Model Conformance. [err:XPDY0130] should not be raised for these.
</quote>

I'm not quite sure whether "defined by" is referring to "limits" or to "primitives"; I don't know what "primitives" means; I can't work out what "that differ" refers to; I'm not sure what "these specifications" are. I'm not sure whether "are implementation-defined" means the limits are implementation-defined or that "these specifications" state the limits to be implementation-defined. Why does it say "and must be documented" - isn't that true of everything that's implementation-defined? And is the "should" an RFC "should"?

Perhaps it would all be much clearer if there was an example of something for which XPDY0130 is appropriate, and an example of something for which it is not.