Lift thickness is the compacted depth of one layer of hot mix placed in a single pass of the paver.
Too thin and the lift won't compact before it cools. The common guideline is a lift at least three times the mix's nominal maximum aggregate size, and four times for coarse-graded mixes. For a 12.5 mm surface mix, three times is about 1½ inches. Too thick and it's hard to get density all the way through, which is why deeper sections go down in more than one lift.
A parking lot is often a binder course with a surface course on top, such as 2 inches of binder and 1½ of surface. Overlays commonly run 1½ to 2 inches. The plan or spec sets the real numbers.
Depth turns area into tons, and small misses are expensive. A quarter inch over on a 3-inch lift is 8 percent more mix across the whole lot. Put the lift on the proposal in inches, repeat it on the foreman's work order, and check it behind the screed.