Stage variables can be used as follows:
They can be assigned values by expressions.
They can be used in expressions which define an output column derivation.
Expressions evaluating a variable can include other variables or the variable being evaluated itself.
Any stage variables you declare are shown in a table in the right pane of the links area. The table looks similar to an output link. You can display or hide the table by clicking the Stage Variable button in the Transformer toolbar or choosing Stage Variable from the background shortcut menu.
You can watch video tutorial of this post here: