Definition of "can" in XBD

[Eric Vought <yyyy@xxxxxxxxxxxxxxxxxxxxxx>]

I have not submitted an aardvark on this yet because I do not understand
what the real definition should be for this document:

On XBD 1.2 (Terminology) lines 9-12, "can" is defined. This definition
appears to be nonsensical. In one breath it says that the feature or
behavior is optional (as would follow from the common English usage of
"can"), then says that the feature or behavior is mandatory and can be
depended on by application developers. The last part of the definition
of "can" does not appear to be any different from "shall".