contributions to a pension

With regards to a pension, if it is underfunded, what would require contributions to be made to the plan? Is the answer to this simply that the IPS would spell that out? I think I’m confused because some examples ask you to assume the plan will make contributions if the funded status gets to this point. But broadly I’m just curious how are pensions regulated in US with regards to their funded status.

ERISA might spell it out.

I’m not sure.