Dependencies are logically connected between fixtures. These can share information on water (pressure, flow rate, etc), lighting or power. Dependencies are elemental, especially for fountain devices.
A classic example: Nozzles connected to valves, which are further connected to a water distributor, which gets finally powered by one or more pumps.
In general, there are two types of fixtures:
Active Fixtures: All fixtures, which are delivering information and are usually DMX controlled like pumps or valves.
Passive Fixtures: All fixtures, which do only have dependency connectors waiting for an input signal. Like nozzles.
Passive fixtures, which are added to the scene, will always simulate input of 100% until they get connected to an active fixture. For example, a nozzle fixture will emit water even if it's not connected to a pump. This behavior makes the scene-building process easier.