An A fund is an unsecured promise to pay an unspecified(due to contract changes, bankruptcy, etc.) sum at an unspecified time(variations in contract. age at retirement, bankruptcy, etc.).

A B fund is paid directly into an account with your name on it on a monthly basis(or more) and is immediately yours. It stays yours despite contract changes, bankruptcy, retirement age, or any other glitch.

People have found over the last decade that no matter how big a company is there is always a possibility of an A fund evaporating before you get anything. See Airlines, GM, Ford, IBM and many others.