Inject - The object creates new fluid with the specified parameters that is injected into the simulation. For particles, the fluid is injected in the position of the particle. For geometry, the fluid is injected within its internal volume. When this mode is selected, the Discharge parameter specifies the volume of the injected fluid.
Brush - The objects update the fluid parameters, slowly bringing them to the specified values. When this mode is selected the Discharge parameter specifies the speed at which the transition takes place.