In general, they are used, as you know, to align body parts/panels as they are bolted on. They enable you to obtain level, plumb, smooth, even sheet metal. They also allow for proper gaps and door opening clearance (doors have hinge adjustments for this also). They are open, horse shoe shaped, slotted as you say so that you can try different thicknesses without disassembling the bolt. It can be a time consuming, be in the right mood, deal. Don't forget that rubber seals, windlace and cab mount pads will affect the fits. As far as body mount hardware (at least on TF) there is a set of bolts, pads, washers and nuts that are used to attach the body to frame. Not shimmed, per se, to align any gaps, etc. You can use regular washers to "build up" if you want, and shims to tweak. The slot widths, of course, tell you sort of where to use them according to the bolt size at a location. You don't use a wide slot with a small diameter bolt.
