There are two types of Dmax.
Absolute Dmax is the highest value density a given photo product will give in a development condition that forms silver metal from silver halide. An example would be treatment in sodium sulfide solution which reduces the silver halide to silver metal. This can range from 3.0 - 5.0
Effective Dmax is the Dmax obtained in any given developer in which the curve and speed are optimum for the film. These two may differ by a substantial amount. This is usually 3.0 - 4.0
There is a third type, related only to print materials, in which Dmax is governed by the reflective support itself and this value is always limited to a maximum value by nature of the reflective material itself and not the amount of silver formed, nor by development. It is usually between 1.8 and 2.2.