It is a bit careless to make a statement like:
"Implementations that include some (but not all) DITA features are considered conforming as long as all of the features that are included follow the requirements that are given in the DITA specification." It could be taken literally for very weak claims of conformance that in fact conflict with more specific conditions stated further. And how do I know that the "included features" are intended or supposed to conform to DITA, as opposed to being just my own extension or proprietary product feature?