I think pretty much any area can have a SrTL. I've seen them in almost every department but AP. It's pretty much up to the STL's discretion where they want the SrTLs (or where they were when they came to the store).
We only ever had SrTLs on the sales floor until AE14. Now we have a SrTL-Replenishment because we lost our ETL-Replenishment and we need more than one LOD overnight. At my store, sales floor makes the most sense for SrTL because TLs from other areas are far more likely to be tied to their own areas (i.e. they may be the only coverage - see: Food Ave, Starbucks, backroom, consumables, etc.).