Runback generally describes an automatic action involving the reduction of turbine load and plant power. At my PWR, we have turbine run backs by certain over power or over temperature conditions..., these "interlocks" trigger an automatic reduction in load in order to hopefully correct the condition before a plant trip occurs. At my BWR, we had loss of condensate flow runback interlocks. If you can't feed the plant enough water to maintain the power that it's at, then you need to reduce load quickly, which is what that runback interlock did for us.
Low pump suction flow/pressure interlocks have to do with protecting centrifugal pumps from a loss of suction. As my PWR recently experienced, a million dollars worth of damage to a pump can occur when it is run without a suction source for long. Typically, these interlocks will prevent you from starting a pump, trip a pump, start another supply pump, etc, to help protect the pump that is experiencing a loss/low suction flow/pressure. At my PWR, our main feed pumps have low suction pressure trip interlocks, and low suction pressure start prevent interlocks. They'll also trigger another condensate pump to start in order to help maintain the main feed pump online.
I hope that helps.
Justin
