Capillary break
A layer of clean open-graded stone or another material that stops liquid water from wicking upward through soil into concrete by capillary action.
What does capillary break mean?
Capillary rise moves water upward through fine-grained soils against gravity, and a layer of coarse stone with no fines interrupts the pore structure that makes it possible. A capillary break addresses liquid water; a vapor retarder addresses vapor, and thorough slab detailing uses both. The same principle applies at the top of footings, where a break protects the wall above from wicking moisture.
In practice
Clean washed stone under a slab is doing double duty as subbase and capillary break, which is why 'crusher run' with fines is not an equal substitute.