In the 5GHz band, only the non-overlapping channels are listed.
Power levels are limited in the lower 4 channels... intended mostly for same-room indoor use.
The mid-band channels can use about 5x as much power, but are still intended mostly for indoor use.
I *think* the FCC opened up some channels in the H-band a few years ago, but prior to that we had only 12 non-overlapping channels in the 5GHz band here in the americas.
The upper band can use 4x more power than the maximum allowed in the middle band (about 20x more than the lower band), and is often used for long-range point-to-point and outdoor dual-band WiFi applications.
I don't think there are any rules that say you cannot use the upper band channels inside, but I'm pretty-sure it's in part 15 that the lower 4 channels are not supposed to be used outdoors.
I think DFS being *required* (to automatically mitigate interference with any nearby airport radar... i.e. you cannot manually select those channels; only tell the device to include those channels as 'potentials') in the middle band came hand-in-hand with opening that band for WiFi use here in north america.
See the attached.
