The back bar is a price ladder
Every bottle in the building sits on one of three or four rungs. Once you can see the ladder, the back bar stops being a wall of labels and becomes a menu you can navigate in your head.
Well (also: rail, house, speed rail). The bottles in the rail under your hands. What a guest gets when they order by category — "vodka soda," "gin and tonic," "rum and Coke." Cheapest per pour, highest volume, and the reason the rail's contents live in your muscle memory.
Call. The guest names a brand — "Tito's and soda," "Jameson neat." Named brands at a mid-tier price. This is most of the branded volume in most bars.
Premium / top shelf. The higher rungs. Aged spirits, single malts, sipping tequilas. Fewer sales, much more margin per sale, and usually kept physically higher and further away, which is where the name comes from.