Re: what is delay cell ?
Usually you have a set of delay cells with different delay values, they are all footprint compatible (same dimensions), This compatibility facilitates timing adjustments because you can freely change any delay cells you use at a given point in the circuit.
They have high drive outputs. This means the actual delay value has a limited dependence on the interconnect load.
Delay cells are rather large. Because of their size, use them with care.
hope it helps!
cheers