It is exactly what it sounds like: every unit in the military has a roster of people assigned to it. The UMR will reflect what the unit should look like when it is fully staffed under ideal conditions. This is key to determining unit readiness and financial allocation.
If a unit is supposed to have, say, 1000 soldiers in it (just to use an easy, round number; battalions can have different numbers of people assigned to them depending on what they’re expected to do) but they only have 500 soldiers, then the UMR will reflect that they are only at 50% strength. If higher command is looking for units to mobilize for a mission, they’ll realize that this 50% strength battalion is not up for the task and will bypass them for a unit more fully staffed… if any are available.
Or, it may be decided that this battalion is the only one that can do the job, and so folks higher up the chain of command will start reaching out to other battalions for qualified personnel to serve as “fills” to “infill” the battalion and bring it up to strength– which was what happened to me in 2004 when I got sent to Iraq.
Having a unit be under-strength and non-deployable means that peoples’ careers are on hold. A unit that is chronically under-strength will cause headquarters to examine them and determine what sorts of factors are hampering its growth.
There are a number of ways to determine how many people should be in a Reserve or National Guard unit. They will look at the population of the town or city nearby; if you’re trying to establish a Brigade-size unit (2,000 to 3,000 troops) near a small rural town of 10,000 people, you’ll never have enough recruits to fill the UMR.
They’ll look at the education level of the nearby town as well. If you have a Reserve battalion requiring very specialized and highly technical STEM training in an area where few people finish High School, then again, you’ll have problems finding qualified recruits.
Local recruiting budgets factor into this as well, and if a population area just cannot provide enough recruits to fully staff a unit, then the unit will be disbanded or relocated to a different population area that can sustain it. Offers of enlistment bonuses and such come into play.
Few Reserve or Guard units are ever 100% fully staffed according to their UMRs– there’s usually a few shortfalls somewhere. Typically these are minor, say, less than 10% staffing shortfalls. But these fluctuate depending on the economy, civilian perceptions of politics, opportunities, and benefits of enlistment, and so on.
But Captain Mizrachi is always eager to add another soldier to the 213th’s UMR, so SPEC-5 Randall Young is added to the list.