Not to be picky, but isn't AMFA an association and not a union?
Implicit in your question is a belief that there's a difference. There is none (aside from semantics).
AMFA is a labor union. The TWU is a labor union (at least in name, despite its impotence at AA in representing the mechanics).
One of the first things amfa did in the UAL agreement was to replace all references to union. The word union was replaced by association in all applicable instances.
A distinction without a difference. The words "Union" and "Assocation" were defined terms. Wouldn't matter what word was used to refer to the union. AMFA replaced the IAM, right? The International Association of Machinists?