It was understood...to be honest I am unsure if it was written policy...probably not in retrospect.
I recall being ASKED as PRN staff to float to stepdown (outside our 3 critical areas)...but it was not mandated... when I declined it was OK.
We were required to float freely to SICU, NICU (neuro) and CCU and were encouraged to gain basic competency in all 3 areas. The scheduling also allowed for PRN's on duty who would be the designated 'floater' in most instances.
Our fulltimers generally disliked floating and would often ask to be canceled rather than be mandated to float. Of course this didn't always fly.
Good luck with your policy...it will be good if you can get one!