There are two options. If all the APs are in a single network you can tag the APs, and then assign a VLAN based on tag. You can see it in the SSID settings. The other option is to put each building into a separate network so you have separate SSID settings. If the buildings do not share a common layer 2 infrastructure (so they are all separate routed domains) then this makes this option a stronger choice.
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