Set the variable: workflow stage upon closure of the task

[Joshua Comeau]

How to write a script in the flow that when the sctask state is set to closed completed update the variable for that task only.

[-O-]

Catalog Task variables are not "for" a given "task only".

Variables are shared with all other tasks of the same parent Requested Item and the Requested Item itself.

You would need to define a separate variable and use UI Policies and Catalog Client Scripts to make sure that a given variable only shows up on the right Task.