IR = Infra Red = must have line of site.
RF = Radio Frequency = works through walls/cabinets/etc.
40.0/52.0/50.0 are ALL RF only when controlling the H3/Joey.
21.x (VIP series) remotes have to be set for IR mode, but then work just fine as long as IR is enabled on the H3. The 21.0 RF tech is not compatible with the HopperX/40/50/52 RF tech.
20.x remotes are IR only and work just fine if IR is enabled on the H3.
You can have any mix of 40/52/(presumably)50 RF remotes paired to the H3 (up to 8 or so remotes total). Since the IR remotes are not paired, you could have as many as you want.
The H3 is smart enough to know which remote you are actively using and changes screen prompts accordingly, ie: displaying "Options" or "App" text in prompts vs "Red" or "Blue" button icons.