Aux battery switch

Still trying to work out the wiring in my old iveco daily... There's a switch in the cab marked aux battery and if I switch it on with ignition on an extra battery light appears on the dash. Can anyone explain what this is likely to mean/do? Thanks

Rambler may well be right..... if so then its very important that the feed is fused......
if not then the light wire will try and become jump leads under starting if the switch is left on and will burn out rapidly.
Best using a voltage sensing control relay, or a split charging relay.

A simple test would to check the voltage on both battery's with the switch off they will probably be different. Then start engine check voltage on engine battery it should rise. And leisure one should not.
Then turn on switch and leisure one should rise also and match the engine one.
If this is the case then be careful not to leave it on,as Jon said if you don't know how its wired it could try to assist the main battery when starting.

Sprinter’s advice is worth doing. It’s worth doing those steps with engine running and then stopped. I’d imagine the switch connects your two batteries, engine running or not but worth double checking. And Witzend brings up a good point about the solar. There might be situations where you want both batteries charging off solar with engine stopped. There might also be times you want to manually connect both batteries, if say your engine battery is flat and you want to crank off the auxiliary. Perhaps too, your auxiliary battery does automatically connect to your starter via a VSR with the switch in the off position and that switch just allows you to connect the batteries manually.