Ok, I am back from holiday vacation now, sorry for the delay.
So this is basically a legacy from when Arsenal went from single job per host to the current "multi-slot" architecture. Now a host is never at status "busy". Some or all of its slots may be taken, but the host is still "ready" to receive commands.
I'd have to review the slave logic, but I think if it has any active assignments it says "Busy" so a workstation user can kill it if they want to work.
hth,
-barry