Emitters are devices associated with junctions that model flow through a Orifice or orifice. In these situations, the demand (that is, the rate of flow through the emitter) varies in proportion to the junction pressure raised to some power. The constant of proportionality is called the "discharge coefficient". For Outlet and sprinklers, the pressure exponent is 0.5 and the manufacturer usually states the discharge coefficient value as the flow rate in gpm (gallons per minute) through the device at a Head Loss of 1 psi.
Emitters are used to model flow through irrigation systems and irrigation networks. They can also be used to simulate leakage in a pipe connected to the joint. (if a discharge coefficient and pressure exponent for the crack or leak joint can be estimated) and calculate a fire flow at the junction (the flow available at some minimum residual pressure) In the latter case, a very high discharge coefficient value would be used (e.g. 100 times the maximum expected flow) and modifying the elevation of the joint to include the head equivalent of the pressure target.
When a normal sender and demand are specified for a join, the demand that EPANET reports in its output results includes normal demand and flow through the sender.