The CDM (or "number 6") sits in front of the defence. In possession he is the first outlet from the centre-backs; out of possession he screens passing lanes into the 10 and tackles in the half-space.
Great number 6s combine positional awareness with short, accurate passing. Busquets, Rodri and Casemiro are the archetypes.
In a 4-3-3 the CDM stands alone; in a 4-2-3-1 he pairs with another number 6 in a double pivot.