CONST_STRING vs string_from_literal ?

[Patrick R. Michaud]

What's the difference between CONST_STRING and string_from_literal?
In particular, which one should be used when?  In some code I see
things like:

    #  src/pmc/exporter.pmc
    STRING *s_hash      = CONST_STRING(interp, "hash");

and in others we have

    # src/pmc/codestring.pmc
    STRING *percent     = string_from_literal(INTERP, "%");


I searched briefly through the docs, source code, and mailing list
archives but couldn't quickly come upon an explanation, so I'm
posting this here.  (Perhaps the answer should make it into a PDD
or other document about accessing Parrot functions from C.)

Thanks!

Pm