You have to map the Input Pipeline variable A to Output 
Pipeline variable B.
Steps
1.   Create any new Flow service
2.   Add some Input and Output variable as required
3.   Insert any previously created service or insert any Map
4.   Click at Pipeline In area then click on the variable A,
5.   Click at PipelineOut area then click on the variable B,
6.   Now Click over link button.

Note: Before this, service at which you are working must be 
locked.