On some shocks (5100s for example), the lower spring seat sits on a c-clip that snaps into a groove on the shock body. There are multiple grooves, ie "perches", so you can move the spring seat up or down to some extent to adjust the ride height. It's essentially the poor mans version of a threaded shock body (no offense to people who have shocks like that). Toytec has a conversion setup which makes it so there is a threaded body on a 5100. The only difference/benefit between a threaded body, and the preset perches, is you get more/finer adjustment, and you don't have to remove the coilover to from the truck to adjust it (versus you have to disassemble the coilover to move the spring perch). The following images illustrates the perch locations and the spring seat on the shock: