I have seen in various places that a margin call requires restoring an account up to
- The initial margin level (or having this done for you) or
- The maintenance margin level.
As best as I can tell, trading on a future's exchange always requires restoring to the initial margin level, while buying and selling stocks on margin through a broker requires restoring to the maintenance margin level. But I have not seen this distinction made clearly or definitively. I have also seen in one place the words "initial margin call" and "maintenance margin call" making me think that either type of requirement might appear in the same context.
Is what I surmised correct, that
1) above always happens when trading futures and
2) always happens when trading stocks?
If so, why is there a stricter requirement for futures margin accounts? Are these just very different uses of the word "margin"? Does it have something to do with the marking-to-market of futures accounts? Does the requirement depend on the type of account, or the class of asset?