This causes the dosing pumps or valves to activate in sequence, one after the other and allows high current valves or pumps are connected to the unit without fear of overloading the controller or its power adapter.
If in doubt about the current required to drive the pumps then always select sequential dosing. If 3 or more nutrient parts are selected sequential dosing is enforced to avoid damage to the controller.
By selecting sequential dosing, the dosing outputs will operate in sequence, one after the other. This allows a small power supply to be used with dosing devices that draw a heavy current. For example, small peristaltic pumps typically draw 1.2 Amps or so at 24V DC.
If three-part dosing was used and three pumps were allowed to come on at the same time the power supply would need to be rated at about 4 or 5 Amps.
By selecting sequential dosing a 2Amp supply could be used.