Can someone define "Reserve". Obviously not "Standby" or "Available". As someone else said; "Sounds more like Standby".
After arrival, of course, you must have min rest. If disturbed within that period, very brave fellas could say so and restart the min rest.
During defined layover, Companies would love to have access to you but it is quite hard to define. A bit of give & take appears to be adopted by most outfits.