Under the current Memorandum, Articles and Laws of GNAS version C-01-03, any club through which you have affiliated to Archery GB is known as your First Claim Club. This means that if you shoot at a County, Regional or National tournament, you must represent your First Claim Club and not any secondary clubs that you have joined as an associate member. Inter-club tournaments or friendlies are different, and you can represent a secondary club if you wish, but only with the consent of your first claim club.
This will remain so for the duration of your current Archery GB membership which usually finishes at the end of September each year. After this, you can elect a different club to affiliate through as your First Claim Club.
A very informative article about the rule is available at Kent Archery's website here:
(Note that Rule 20(b) is now GNAS Law 25(e) under the current version of GNAS laws)