The B777 does not have an A/T arm mode similar to the B737.
Boeing does not recommend the arm mode due to the "mode confusion" argument and secondly an increase risk of tail strike.
I have read reports on two incidences of mode confusion caused by A/T arm and could certainly see how Boeing came to that 'all on or all off' conclusion. If fact our flight ops management changed from A/T arm to "all off" after those incidence.
The tail strike risk was demonstrated to me once and I was not that convinced about the likelihood but that was with the best of the breed. The B733.
Interestingly it is called both alpha floor protection and minimum speed protection in my FCOM.
The problem with FCOM is that Boeing produces a "tail generic" FCOM for the operator that is supposed to cover the tail specific serial number jets in an operators fleet. From this generic FCOM the operator then customises the FCOM for its own operation. That is why you get all the weird statements like "alpha floor" because some Chief Pilot thought it was better to call it that.
If your operator has its Airline name at the top of the FCOM then it is a customised version.