Meaning of keyword category list in src/backend/parser/gram.y

Hi

I'm trying to add a new constraint to column constraint.
I add a new keyword ro kwlist.h and gram.y, and also edit ColConstraintElem.

The problem is i don't know what is "Keyword category lists" at the end of
gram.y.
when I added the new keyword in "unreserved_keyword", shift/reduce and
reduce/reduce conflicts appear in bison outputs.

any idea about this problem and meaning of "Keyword category lists" is
appreciated.

Cheers
Soroosh