Many IP controllable devices feature advanced commands like that. There isn’t a simple rule for how to use each one, and some of them are too complex to try to document such as .PANDORA SET. However, some of them we do document and discuss and those can be used freely. For instance we’ve previously documented the .AIRPLAY SET and the new . AIRPLAY TOGGLE SET command in 1.7. Those just take simple Airplay speaker names.
Roku .CHANNEL SET is slightly less friendly, but not difficult. Use a web browser to access:
Replace 18.104.22.168 above with the IP of your Roku. That will show you the channel next to the ID of the channel that you would enter into the .CHANNEL SET command.
In general there is one rule. If you look at the command in Roomie and it asks you for a specific entry with a placeholder, for instance .VOLUME SET always asks for a normalized volume value in standard format (eg. ‘-42.5’), then the command accepts those friendly values. Otherwise the command is device native, and requires you to know what that device itself is expecting, for instance the Roku channel ID above, or a Denon radio preset, or countless other examples. You can always ask support if you need to perform a specific function.