A small bug, but none the less an annoying one.
Join Strings Input list lists the variables in order of creation. Where as Join Strings Output list lists variables in alphabetical order. Both lists let you select the proper variables, however it take much longer to find the variables due to them not being in the same order.
Suggestion: Perhaps have them inherit from the setting of the Recorder Variables list, so it displays the variables in the same order as in the Recorder Variables.