Flow functions
Using the Flow functions, you can specify a value for each of the flow variables defined for a flow, and additionally, alter an already-specified value for a flow variable, if required. Flow variables act as an alternative means to pass information through and through a flow instance—thereby eliminating the limitations and cumbersomeness of nodes being the only gateway for passage of information.
Flow variables are name-value pairs that persist until the flow-instance that is using these variables is executed. Flow variables are case-sensitive. So, you need to be particular about the case you are using while defining a flow variable. Flow variables are also instance-specific; that is, no two flow-instances that are running simultaneously can share the same set of variables. However, a set of flow variables defined for one flow instance can be used by another as long as the other instance is not executed during the runtime of the instance for which the variables are originally defined.
See the Flow variables documentation contained in the Functions and Expressions sections of this online documentation.