This is the flash protection port. It controls whether the flash is locked or unlocked. Flash can only be written to, erased, or protected pages read if it is unlocked. The input of this port is 0 or 1, 0 if it's locked, 1 if it's unlocked.

To unlock the flash output 01h, to lock output 00h. Unfortunately output to this port is only accepted from code executing on a limited number of ROM pages, namely 1Ch, 1Dh, and 1Fh (possibly 1Eh but that's not a code page anyway) on the 83+. Those are pages 7Ch, 7Dh, 7Fh on the SE.