A partial termination may be deemed to occur when an employer reduces its workforce (and plan participation) by 20%.
This may trigger a Partial Plan Termination.
If your plan has a vesting schedule and associated employer contributions those affected employees may become 100% vested.
This affects your forfeture account and anticipated payment of expenses or offsettign funding of future contributions.
Plannig ahead for this contingency and having a strategy if it occors can have a material difference in its success.
Here is a good article that discusses the basics of it.