Fan-out:We can use the Fan Out primitive to fire the output TERMINAL once (with the input MESSAGE) or to fire the output terminal MULTIPLE times. You can use Fan Out in isolation or as part of a Fan Out and Fan In combination.

Fan-In: Fan In is always partnered with a Fan Out in the same flow and acts as a decision POINT for when to continue flow execution. It receives a number of messages until a decision point is reached, at which point the last message to be RECEIVED is propagated to the output terminal. The Fan In primitive may only be used in combination with Fan Out. 

Fan-out:We can use the Fan Out primitive to fire the output terminal once (with the input message) or to fire the output terminal multiple times. You can use Fan Out in isolation or as part of a Fan Out and Fan In combination.

Fan-In: Fan In is always partnered with a Fan Out in the same flow and acts as a decision point for when to continue flow execution. It receives a number of messages until a decision point is reached, at which point the last message to be received is propagated to the output terminal. The Fan In primitive may only be used in combination with Fan Out.