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The W3C XProc specification does a great job of differentiating between implementation-defined and implementation-dependent features, with a convenient list of each in the appendix. The W3C XQuery 1.0 spec (http://www.w3.org/TR/xquery) and XQuery 1.0 and XPath 2.0 Functions and Operators spec (http://www.w3.org/TR/xpath-functions) similarly defines and delineates implementation-defined features.

Appendix A (http://www.w3.org/TR/xproc/#conformance) contains the following text:

Conformant processors must implement all of the features described in this specification except those that are explicitly identified as optional.
Some aspects of processor behavior are not completely specified; those features are either implementation-dependent <http://www.w3.org/TR/xproc/#dt-implementation-dependent> or implementation-defined <http://www.w3.org/TR/xproc/#dt-implementation-defined> .
[Definition: An implementation-dependent feature is one where the implementation has discretion in how it is performed. Implementations are not required to document or explain how implementation-dependent <http://www.w3.org/TR/xproc/#dt-implementation-dependent> features are performed.]
[Definition: An implementation-defined feature is one where the implementation has discretion in how it is performed. Conformant implementations must document how implementation-defined <http://www.w3.org/TR/xproc/#dt-implementation-defined> features are performed.]

Section A.1 then lists all the implementation-defined features, and section A.2 lists all the implementation-dependent features.

I think the XProc spec provides a great example to follow on two counts. First, it formally distinguishes between implementation-defined and implementation-dependent features. The choice of terms isn’t nearly as important as the distinction itself, of course: implementations must document how certain features are implemented. In the DFDL realm, section 2.6 lists some implementation limits which always constitute schema definition errors; surely these are the types of details that must be documented by any DFDL implementation. Using terminology such as “implementation-defined” and “implementation-dependent” would flag these types of documentation requirements for implementations within the specification.

Second, all the implementation-defined and implementation-dependent features are listed in one place in the specification. I think doing the same in the DFDL spec would provide a great resource for DFDL implementers.