This is a special emitter to abruptly fill the volume of an assigned mold with liquified particles. Unlike the other particle emitters, a continuous flow of liquid particles is not possible. The classic running tap should therefore continue to be simulated with a Basic Emitter, for example.
The advantage of the Liquid Fill Emitter is that it produces the liquid particles within a volume in fixed grids, which means that initial overlapping of the particles can be avoided. These particle collisions can otherwise lead to unwanted explosions of the simulated particle flow and are avoided here. This function also distinguishes the Liquid Fill Emitter from the otherwise similar Mesh Emitter, which can also be used to generate particles within an object volume.

When creating liquid particles, please note that this also automatically creates new, custom properties, which are described here in the Simulation Settings. This allows all relevant properties of the liquids to be read out and readjusted at a later date using the Conditions and Modifiers of the standard particle system.
A special Liquify Modifier is also available, which can both change the properties of the liquid and allow conversion between standard particles and liquid particles.