The Triple Band Stack will simply remember (store) the last frequency entered or tuned for that band and stack position. Try this:
- pick a band - 40 meters - by pressing [7.0]
- Tune to a frequency
- Press [7.0] again and tune to a different freq on 40 meters
- Press [7.0] again and tune to yet a different freq on 40 meters
Now, press {7.0} repeatedly and you will cycle through the last 3 tuned (or entered) frequencies on that band. This is the triple band stack — there is no formal store command or button for the triple stack. The [STO] feature is for storing frequencies in Memory/Channel Groups.
The Triple Band Stack will not only store the frequency - it will store mode such as LSB or CW. Both VFO A and B have independent Triple Band Stacks!
I believe this is explained in the manual under “Convenience Features”.