Interesting question. I dont think a saveset will be modified when it is use.

And what if it is not used yet, but will be needed later when the current saveset is done?

If not used, and if eligible for recycle, it will be marked so (in case of disk devices, it will be removed). 

Dag wrote: And what if it is not used yet, but will be needed later when the current saveset is done?

So the current clone operation will fail when it reaches that saveset then?

Dag wrote: So the current clone operation will fail when it reaches that saveset then?

Read again what you asked.  You asked:

Dag wrote: And what if it is not used yet, but will be needed later when the current saveset is done?

If it is not used that means it is not required by any current operation (like nsrclone or nsrstage).  If you were referring to nsrclone/nsrstage where you have array of ssids {n, n+1, etc} and you wonder if while processing n+1 (clone or stage) will {n+6} be left intact by nsrim process given that is being used by current operation, then I do not know the answer from practice, but in theory it should not be touched. In past, nsrim would skip such save sets if they were in use or pending to be used.  Since the past I refer to, many things have changed in workflow and certain things have changed surely, but it is safe to assume that engineering had to think of this as one could easily get into race condition where data would be (re)moved and affect ongoing/current operations.