the CAN-BUS plug is only used if you add in a CAN-BUS device like an iOSD Mini or I think something about the Groundstation might use it.
Most people that put in a FPV system go ahead and move that CAN-BUS connector up inside the Phantom body anyway. no, it isn't flight or operation critical you can remove it or just move the plug up inside the body (you disconnect the end bit, pull the cable up, then reconnect the end bit) if you might install FPV later
There is another attached-to-leg device, which is the COMPASS. it's the thing attached to a leg but there's no other connector.
the compass IS flight critical so you need to make sure you reattach that.