alfa and SoCal-
Excellent explanations.

alwaysflying-
"A,B and C" funds are not legal terms. Its just contract shorthand.
The exact meaning varies from airline to airline.
Delta has a "C" fund. Its called DPSP. Its a 401k. Separate from the defined contribution "B" plan. At most companies this is the fund where the company "matches" the employee's contributions of pre-tax income up to a certain amount. Delta does not match, but instead just gives everyone a flat 2%. The employee can also contribute his own money up to the IRS limit or nothing at all if he so chooses.

We use a lot of jargon and acronyms in these threads that must make it hard for people new to the industry to follow. Maybe we need a separate thread for definitions and explanations of these terms and where newbies could ask questions. I nominate alfaromeo as moderator.