ARM "single band" is the default and what you want to use.
"Multi Band" is from a long time ago when we sold access points that only had a single radio. Multi-Band would allow an AP with a single radio to be tuned to 2.4ghz or 5ghz, depending on RF conditions. "Single band would allow a single radio AP to only tune to the parameter in the AP System Profile under "RF Band", which is typically g. Aruba/HPE does not sell any single radio APs anymore, and "single band" and "multi band" has no effect on multiple radio APs.
To recap- single band and multi band only have an effect on single radio APs, so you don't have to worry about them.