Why the dust in the mix matters
Clean stone, the kind you see used for drainage around a foundation or along a footing, is washed so there is nothing but rock in the pile. 21A stone is built the opposite way on purpose. It keeps the crusher dust and the smaller fractions mixed in with the larger stone, and that dust is what fills the gaps between the bigger pieces when a compactor runs over it. Clean stone stays loose no matter how many passes you run over it, because there is nothing to lock the pieces together. 21A stone packs down into something closer to a solid mat.
That difference is why 21A, not clean stone, is what typically goes under a driveway or a slab, while clean stone still has its place around footing drains and other spots where water needs to move through freely instead of a surface needing to bear weight.
How it gets used on a base
Once the topsoil and soft ground are stripped out, 21A stone gets spread over the subgrade and compacted, usually in more than one layer rather than dumped in one thick lift and rolled once. On red clay ground, that compacted stone layer is doing real work. It gives the slab something firm and free draining to sit on instead of resting straight on clay that swells and moves with the weather.
How thick that stone layer needs to be depends on what the ground looks like once it is stripped, and how much support the slab above it needs to carry. That gets decided on site, not off a phone call.
How it differs from subgrade and subbase
It helps to keep the layers straight. Subgrade is the native ground itself, whatever it was before anyone touched it, usually clay around here. Subbase is the general term for the compacted stone layer that sits on top of that native ground and under the slab. 21A stone is one specific spec for the material that fills that subbase layer, not a different layer of its own.
That distinction matters when you are talking through a job, because a soft subgrade and a thin subbase are two different problems with two different fixes, even though both of them show up as the same thing later: a slab that settles or cracks where it should not.