You should use the ANT+ FE-C option. Zwift does weird things with pairing screen. Instead of using what was paired the last time you rode, it seems to want to rescan and pick whatever and/or pair something that wasn’t paired at all the previous session. As such, you might need to unpair, search, and pair what you want before each session.
Long story short… Prior to ANT+ FE-C, many of the big trainer brands (e.g., Wahoo, Tacx, CycleOps, etc…) all used ANT+ to control the trainers. However, it was more their own version of ANT+ so to speak. Apps like Zwift, in order to support different brands, would have to deal w/these different versions of ANT+. ANT+ FE-C is a standardized version that saves the app makers a lot of work. If all the trainers are using the same protocol, app makers only have to write code to support that protocol vs special/modified code for every brand.