I installed these through high school and for a couple of years after I got out of the Navy.
They go everywhere, period. Crawl spaces, enclosed areas under stairwells, cement rooms, crawl spaces, etc. No more than 15' between heads and no closer than 7.5'. If you get too close to a wall, a sidewall sprinkler head is used. (Like you see over hotel doors.)
If the public uses the building, you can bet they are in every nook and cranny. If there is more than a foot in deviation, like a 14" beam, in ceiling height that area is a different hazard zone and must be totally covered as well.
BTW, if we could get permits not to cover an area we would because sometimes it costs more to run a line (or enough water to an area) to place a head. For a bid, you can estimate between $75-150 per sprinkler head to guestimate how much a system will cost. (It may sound like a lot, but that takes into account getting water from the street, installing risers and an intricate valve system, all the piping you don't see, and the labor.)
Go on, ask why there are heads over restaurant cooktops and fryers...