The term identifies the defining meat on an individual sandwich and does not characterize a meat on it's own merits. For example, in a BLT bacon is the primary meat; on a turkey club, bacon is not the primary meat.
The Primary Meat on a sandwich may or may not determine the name of the sandwich:
A Turkey Club is named for its Primary Meat, but an Italian Club is named for the collective ingredients of the sandwich, not a singular Primary Meat.
-Tiebreaker 1: When multiple meats are on a sandwich, as with a club (ham, turkey, bacon), the highest quantity meat becomes the Primary Meat.
-Tiebreaker 2: If multiple meats are present in the same quantities, the meat which provides the most flavor becomes the Primary Meat. In our example above of a club sandwich with ham, turkey and bacon, if there are 2 fingers of each meat, then bacon would become the Primary Meat.
(Exceptions to this case would be flavored meats such as cajun turkey, smoked ham)