MSA can mean "Min Safe Alt" or "Min Sector Alt" and unfortunately, dont mean the same thing.
The Min Safe Alt version relates to the 25nm circle, with ROC additives 1000/2000 depending.
Min Sector Alt is usually a subset of the Min Safe Altitude. These are frequently noted on the charts with the individual sectors/arcs noting different minimums per sector, rather than the overall Min Safe Altitude...which may be far too restrictive for operations.
In regards to SSA, I have not heard of that...but well....