Single sided uprights are for attaching accessories to single workbenches. These are the "standard" style of uprights.
Double Side Surface Mount Uprights
Double sided uprights are for attaching accessories to two benches placed back-to-back, without using two sets of uprights.
Single Sided Leg Mounted Uprights
Leg mounted uprights are for use when attaching the uprights to the surface is not possible or undesired. Bolts directly to the table leg.
The "standard" uprights come in two styles, one for laminate or phenolic benches, and one for thicker butcherblock surfaces. Surface mount uprights are also available in Zinc-Plated, making them ESD appropriate.