That outputs, that is A, B, C, and D, are paralleled at isolation into separate DMX drives for the 5 pin and 3 pin. The DMX Thru is paralleled after the DMX drive and thus can't use the 5 pin and 3 pin output at the same time. Likewise, the input is paralleled. So you can use all 8 connections on the front, four 5 pin and four 3 pin, but 5 pin and 3 pin aren't isolated on each section, be it A, B, C, or D. The sections are isolated from each other.
It is also important to note that if something takes out/effects the line on the DMX Thru, it will take it out/effect it all the way back to the console. This is why they say only use other splitters on the DMX Thru.