This port is only used on the SE. Well, it exists on the 83+, but it's not anything that's used as far as I can tell and I don't quite know what it does. Anyway, on the SE:

This port controls which page is swapped into the C000-FFFFh memory area. This was previously thought fixed at RAM page 0. Only RAM pages may be swapped. On the SE, there are 8 RAM pages (16 KB each, total of 128 KB). The normal ones are 0 and 1, with 1 being 8000h-BFFFh usually, and 0 usually being C000-FFFFh.

Reading this port will return a number between 0-7, which is the RAM page in C000-FFFFh. You can output values 0-7 to this port, to swap in that RAM page.