org-tag-alist

org-tag-alist is a variable defined in `org.el'.
Its value is

nil

Documentation:
List of tags allowed in Org-mode files.
When this list is nil, Org-mode will base TAG input on what is already in the
buffer.
The value of this variable is an alist, the car of each entry must be a
keyword as a string, the cdr may be a character that is used to select
that tag through the fast-tag-selection interface.
See the manual for details.

You can customize this variable.