Vacuum, gravity, and pressure fillers use a source of power whether it is vacuum (to suck), gravity, or pressure to deliver liquids to multiple nozzles from a single source (tank, manifold) of product. They control the fill either with a nozzle that will overflow to a collector, when the product reaches the right level in the container, or through a nozzle that closes after a preset but adjustable time.
Typically these are the least expensive fillers, each has its pros and cons, with the vacuum handling the thinnest liquids into bottles that will not collapse when vacuum is applied to it through to time pressure fillers that can handle free flowing but thicker products.
Issues with these technologies involve having to collect and recycle overflow, only having a fill to a level rather than to a correct volume, and having issues with fill accuracy on time pressure fillers. Time pressure fillers require strict regulation of the pressure, time and product consistency (temperature/viscosity), a slight alteration of any one would affects the fill.
Usually these systems are used with thinner rather than thicker products and with the less expensive products such as water or cheap chemicals. (Accuracy not an issue)