Attributes are mapped into action nodes as Input Parameters ONLY IF WE ARE GOING TO PROCESS THEM IN THAT NODE and out of action nodes Where attributes require processing by an action node, e.g. in an expression, the attributes must be mapped in as Input Parameters. These attributes are then mapped out of the action node as Output Attributes.

Connectors between the action nodes then pass through the data and attributes DATA AND ATTRIBUTES?. Every attribute is passed through* but in order to use an attribute in a specific action node it must be mapped in to it as an input parameter. CAN WE TIDY THIS PARAGRAPH UP, I THINK IT CAN BE MORE CONCISE AND CLEARER*although pass through every attribute*, including any not mapped**. 

*attributes not mapped are referred to as pass-through attributes.

**there can be exceptions, such as the e.g. Converge action.