FASST is a brand name for one of Futaba's 3 implementations of the 2.4 GHz system. They also have FHSS which is their low end implementation.
Futaba FASSTest is an extension of FASST that includes telemetry. I believe Futaba FASST radios can talk to FASSest receivers but they can't take advantage of the telemetry features.
FASST radios can not talk to Futaba FHSS receivers and their FHSS radios (6J and 8J) can not talk to FASST or FASSTest receivers.
Note that FHSS is also a generic term meaning Frequency Hopping Spread Spectrum. But just because a radio says it uses FHSS does not mean a Futaba FHSS readio can talk to it.
It gets even more confusing when you get to modules and DIY RF adapters that you can solder inside your radio. I have a Futaba Radio that was originally a 72 MHz radio. Then I added a Spektrum DSM2 module. Then I added a Futaba FASST module. Then I added a Hobbico AnyLink module. Now I can talk to a variety of receivers.
Unfortunately this 2.4 GHz world is a mass of conflicting and confusing terms. Ya just have to keep asking questions. Those of us who have studied this stuff will be glad to help.
Are you looking for receivers that will work with your radio? What radio do you have?