Groups are used for configuration of APs/Switches/Gateways and sites are used mainly for visualisation/reporting/monitoring based on geographic locations.
So as long as the devices have the same configuration (more or less, you can get ways with having one group.
Realistically if you have a lot of sites, they could be categorised by size like small/Medium/Large and most likely, they could have different configurations and hence the use of different groups.
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Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba.
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