This is where the game keeps track of if a unit has traded this turn.
0202BBf5’s 0x2 bit: Has the unit traded this turn
Check at 08021E20
When you press B on the menu, it decides if you should try to canto(if you have it) by loading this byte at 0801CB06 and checking it against 0.
Presumably, the other bits in the byte tell about other actions that force Canto but not exiting the menu. Ride/Exit/Talk/Support/Armory/Vendor/Secret/Arena/Merch?