Surface regularity - departure from datum
Level to datum is a measure of departure from a fixed plane (datum) specified by the designer. The tolerance is usually ±15mm about that plane. In general the actual level of any edge formwork and the concrete placing and compacting process will influence the level accuracy of the finished surface.
Tighter tolerances than ±15mm should not normally be specified as in practice they are very difficult to achieve. Taking levels on the intersection points of a 3m square grid is the preferred method for checking level to datum tolerances.