This sounds like a really dumb question. Obviously offline means the users app is not running, likely because their PC is off.
Except, that doesn’t appear to be the case. I have four users in my favorites that unless they are on a call always show offline. They are part of a queue, and those calls will route to them even though they are ‘offline’. They’ll answer the call going straight from offline to on a call and then directly back to offline when the call ends.