Buildings = we use as actual buildings (19 defined)
Departments = I look at this one more as "deployments". For instance "High School MakerSpace", "High School Science lab", "Music Labs", "Radio TV labs"...mainly when I see department in our JSS, I imagine what deployment the Macs or iOS devices are a part of.
Network segments= we use them and have every single one of them defined, but they aren't something I can rely on too much. For instance "Music Labs" could stretch across two parts of a building, which may be on two different logical subnets. Hopefully you have a small number of subnets on your internal network, they might be useful to cover a part of a building or maybe a whole building in your case.
Sites = I attempted to use them but now regret that we did. So we have a special education cooperative and a career center that we wanted to declare as two separate sites with what was originally thought as someone outside Central IT's control providing day-to-day support. As such, Sites looked attractive to handle that and we would only have to minimally be involved. Eventually the needs of both got to the point that a dedicated person in Central IT under our control was actually hired that had great experience working with me on our JSS. We still have sites, but I wish we didn't given our current support reps.
