For some reason, there are a lot of Microspeak terms related to personnel. (Maybe you folks can tell me how common these terms are outside Microsoft.)
We start with a term that is not actually used much outside the personnel world: The Position Control Number, or PCN. The PCN represents a place where an employee could be hired. If somebody is actually hired for the position, then the PCN is filled; if not, then it is unfilled.
The term you are likely to hear outside of the personnel world is headcount. (Pronounced as the two words head count, accent on the first word.) This is a filled PCN, and it is often abbreviated to just head.
Another term you are likely to hear is a req, short for requisition, and pronounced like the word wreck. A req is a requisition to recruit; in other words, it is permission to look for somebody to fill a position.
We have an open req to find somebody to frob the whatsit so it can futz the doodad.
An open req is a req that has not yet been filled. This sounds redundant to me, because a req by definition is unfilled, isn't it?
Yet another personnel term you may encounter is backfill. This refers to hiring someone to take over a position that has been vacated by somebody who left the team. You will sometimes hear the term used in a metaphorical context.
Who is the backfill for Bob while he is on vacation?
Bob has not actually left the team; the person merely wants to know who is covering Bob's responsibilities while he is on vacation.
The last term I'm going to expose you to is the ROP, or Recruiting Only Position. A ROP is permission to interview someone for a position that doesn't exist yet. You open a ROP with a particular person in mind, and once obtained, you have permission to interview them. You can think of a ROP as unapproved headcount, since if you decide to hire the person, you still have to find a PCN to put them in. And if you decide not to hire the person after you interviewed them, you close the ROP.
I have no idea how useful these terms are for people not in the personnel world, but I figured I'd write them down for my own benefit, so I have something to refer to when I run across them.