Crudely described, it's a stack of 1" jo blocks wrung properly together to stack up with very little error. The entire stack is on a precision lead screw that drives a full 5 digit DRO (if I remember right, but pretty sure it's 5 full digits). It can be used for several things, mostly old school now days. You can set bore gages with it, if you have the accessory kit, you can use it to calibrate height gages, as well as transfer or compare measurements. It's kind of like stacking up a handful of blocks and using a height gage with a DTI to do comparison readings against parts under inspection, except it's adjustable. You can use it to cal/cert dial indicators and a few other instruments. There are other uses, but perhaps more obscure or I have forgotten them. In the old days, every inspection department had one.