underfunded plans

[fidu]

Seasons Greetings all.

Can someone give me the short version of underfunded plans and make up contributions.

first, when is it determined? year end? by plan actuaries? for which type of plans typically? and which plans have no underfunding requirements.

secondly, if it is determined, (what is the timing test, at anytime?, or as of plan year end?) that the plan is underfunded, what steps are to be taken, within what time frameframe. is there a reporting requirement? penalties assessed?

what governs? pbgc? dol? irs? common sense (just kidding on that last one)

happy healthy and prosperous holiday season and new year to all ya.

Thanks as always.