If you set any variable in the transformation you can not be used those variables in that transformation. Because all steps in a transformation run in parallel without a certain order of execution.
So, the rule is- you can set the variables you want to use in the first transformation of JOB.
Note:- If you set variables inside transformation, you can use those variables from job.
I have prepare a sample transformation & job for you HERE. You can get full idea from log after run the job where I have set 2 variables with same name 'week_start' in both transformation and job.
In short, if i want to give your answer then that will be "when you execute the job then value you use for job -will be/need to be use inside transformation"
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