The difference between A-Trac and TRAC is that A-Trac is for 4WD and TRAC is for 2WD.
Here's a definition of each that I found on the Toyota Canada site:
A-TRAC
Benefit: Helps prevent drive wheel spin on slippery surfaces and in off-road. This system applies brake fluid pressure to the slipping wheel, distributing the drive force that would have been lost, to the remaining wheels. At the same time, engine power is reduced through the Electronic Throttle Control System (ETCS-i). This system applies to all 4 wheels and eliminates the need for front, centre or rear differential control (Locking or Limited Slip) in most cases. A-TRAC functions as 4-Wheel Traction Control.
TRAC
Benefit: Helps prevent drive wheel spin on slippery surfaces. When sensors detect that one of the driving wheels is about to spin, TRAC applies its particular brake and - if necessary - also reduces engine output until the spinning stops and the wheel regains traction. By applying the brake, the torque is sent to the opposite wheel, thus acting like LSD.
In Canada, the 2008 Sequoia is NOT available in 2WD so the brochure only talks about A-TRAC. There's no mention of TRAC, so I would assume that a vehicle would have one or the other, but not both.
I'm going in to take another look at the Platinum tomorrow so I'll check to see if there's any way to disable it (but I kinda doubt it).
Edit:
I just re-read the other thread you started and saw that eddeeeee had
answered your question already. It's good that it can be disabled because it gives more control to the driver. My old Sequoia could disable the VSC while off-roading, but not the traction control.