The textbook treats an unspecified general expression as definite in indication. Once a valid specification has removed some of its scope, a specified general expression is open to further substantiated specification and is therefore treated as presumptive in that respect.
Imagine a general instruction governing ten persons. If valid evidence removes two, the instruction remains for the other eight. Further evidence might, in principle, require another exception. These numbers are purely pedagogical; they are not a legal ruling.