FSSE is one of the many "Maths Functions" - a list of which can be found with the Maths Functions Icon (next to the APL Keyboard Icon on the top pallette).
FSSE is used to copy the contents from one stream to another, using a multiplier factor.
The three parameters required are the DestinationStreamNumber, the Factor, and the SourceStreamNumber. I remember these as D, F, and S (or DFS as in Definitive Feasibility Study).
Using a FSSE in CtB for a unit operation enables you to have multiple parallel trains with the same input stream, and by using the multiplier fact F appropriately you can preserve the mass balance. FSSE can also be used to run a number of simultaneous "what if" exploration studies on the effect of a range of parameter values on a unit operation.
FSSE is partucularly useful when working with size distributions and multi-component modelling.