what does DC Com mean?
Those are the ground pins.

Even process of elimination leads you there. All have a listed voltage except pin 8 which is N.C. (not connected). 

It's feasible that the ±12V and ±5V supplies are generated from separate isolated supplies without the grounds being internally connected. You could check the other side of the connector to see if they're connected on that side.

But, either way, jumpering them together should not be an issue. I base that on the fact that they didn't label them as separate nets.