Which service is it that you are trying to stream? The Calls platform only supports non-trunked (Conventional) frequencies when feeding using Trunk Recorder (Conventional) or VoxCall setups, where you will be given the option to list the specific frequencies you will be streaming. If you are selecting any other option, you won't see the Frequencies box, just the ability to select trunked systems that are already in the RR database.
"Other" will only appear as a choice when applying to host a standard live audio feed. Provided it is not one of the prohibited service types, and does not fit in the listed categories, Other would be a choice that shows where your feed type would be listed when searching for feeds using the filter options.