Are subscriptions mutual? If I subscribe my commlink to a drone, does it use up a subscription slot on both devices or just on one? I'm just trying to get a better idea how Matrix devices actually communicate with each other.

I would say "yes," but that's without actually looking anything up.

I agree with Malachi. Subscriptions are two way streets, due to the nature of communication feedback. If I link into a device, I am not just watching the broadcast output from the device, my system is sending back information confirming the input is correct. Think of the difference between cable TV and cable Internet. With TV, the broadcasting system doesn't care what you receive, doesn't check on accuracy, and doesn't spend time with corrections. Internet, however, sends information out, your computer receives and sends a confirmation, and the sending system confirms the packets were correct.

and just for the record, unwired introduced a secondary connection called a data request, so that one do not need a subscription jost to watch the morning news...