Flow Rate is the flow rate through the fan. It should be used to size the system fan, which should have a flow rate of at least this value.

Velocity Head Blah blah

In-take Pressure Drop is the maximum pressure drop on the in-take side of the system. It should be used to size the system fan.

Out-take Pressure Drop is the maximum pressure drop on the out-take side of the system. It should be used to size the system fan.