[FT] Meaning of FTST0008 and FTST0018

From the specification section 3.4.3:

> If the URI specifies a thesaurus that is not found in the statically known thesauri, an error is raised [err:FTST0018].

and section 3.4.7:

> If the URI specifies a list of stop words that is not found in the statically known stop word lists, an error is raised [err:FTST0008].

I don't understand why URIs for these things would be "statically known."  If the user want to specify some arbitrary URI that points to a valid thesaurus or a valid stop-word list, why shouldn't that "just work?"

Does "statically known" mean that the implementation has a set of hard-coded URIs that the user can only select from?  If not, then what does "statically known" mean here?

- Paul