Variable: minibuffer-completion-table

Alist or obarray used for completion in the minibuffer.
This becomes the ALIST argument to `try-completion' and `all-completions'.
The value can also be a list of strings or a hash table.

The value may alternatively be a function, which is given three arguments:
STRING, the current buffer contents;
PREDICATE, the predicate for filtering possible matches;
CODE, which says what kind of things to do.
CODE can be nil, t or `lambda':
nil -- return the best completion of STRING, or nil if there is none.
t -- return a list of all possible completions of STRING.
lambda -- return t if STRING is a valid completion as it stands.