Preset: with this setting you allocate a value to the flow. This could be by typing a value into a parameter associated to the connector or by a formula in the family. You would normally use this with terminal fittings such as air terminals, radiators, fan coil units etc. The Preset value is then pushed into the connected pipe/duct.
Calculated: with this setting the connector reads the value from the connected pipe/duct and automatically populates a parameter value. You would normally use this with items like pumps, fans, boilers, chillers etc. So the values of the Preset connectors at the terminals are pushed into the pipes/ducts, these totalise through the system and then the total value is pushed into the Calculated connector at the pump/fan/boiler/chiller etc.
System: acts the same as Calculated but used where you have multiple items of equipment in parallel, e.g. two chillers which share the overall duty. When the System option is used a flow factor setting will appear and this is the proportion of the flow handled by each of the parallel items of equipment e.g. two chillers each handling 50% of the total would have a flow factor of 0.5 each.
Some families might have different settings on different connectors, for example a water heater would have a Calculated connector on the Domestic Hot Water connector (which reads from the pipe the total hot water flow required by all of the Plumbing Fixtures), whilst the Domestic Cold Water connector would be Preset and will be equal to the the Calculated hot water flow by means of a formula.
The attached document is a good starting point as a learning resource.
