The tmode of "TSQL-R" is not documented. It appears to be used in a few drivers to map to the Yaesu internal squelch mode of "REV TN".
Many (most?) radios have a reverse tone mode, several call this TSQL-R (or similar) in the radio itself and that's what the convention in chirp is. Many drivers in chirp may not support it, but that's a different thing.
Is this correct? more generally, is it acceptable to add an obscure mode to the UI? I had originally added "REV TN" as a value for an "extra" memory field on the ft-4. The FT-4 has yet another "extra" sql_type called "PAGER". can I add "PAGER" to the UI tmode list instead? I feel like both "TSQL-R" and "PAGER" should NOT be in the standard UI tmode list, but I will defer to the experts. My reasoning: the UI should remain as generic as possible to maximize the sharing of channel lists. Treating this sutff as "extra" allows the driver implementer to decide how to handle this stuff on a case-by-case basis. I'm als not completely sure that the semantics of "TSQL-R" is the same for non-Yaesu vs. Yaesu, although I think it's the same for all Yaesus.
TSQL-R _is_ part of the core and should be exposed as such if you're going to add it in your driver. Same for DTCS-R.
Many drivers have a "Pager" or "Bell" setting too, but very few drivers in chirp support it and it's not modeled in the core. So, I'd prefer you model your pager setting as a mem.extra, and support TSQL-R for the reverse tone function.
--Dan