Units of Bloodletters, Daemonettes. Horrors, and Plaguebearers are True Core Units.

Each unit of these you include in your army allows you to field one other (non-True) Core Unit, one Special unit and one Rare unit of the same Greater Power.

For example, a unit of Plaguebearers allows you to field a unit of Nurglings, one of Plagueriders and one Chariot of Nurgle.

Two units of Plaguebearers allow you to field up to two units of Nurglings, up to two units of Plagueriders and up to two Chariots of Nurgle.