How to filter BW Variable Selection values?

Hi all,

When running a query with a selection variable, I need BW to filter the available values in the variable selection screen for a specific characteristic, based on a specific criteria. As an example, the values which are available for selection on "Sales Rep" needs to be filtered based on the business area the Sales Reps belong to. Alternatively, the "City" values available for selection need to be filtered on the country linked to the person running the report.

Using pre-calc, buckets or user exits only affects the values eventually selected, not the values available for selection. I understand that using BADIs is the way to go? If so, please assist with understanding how I need to approach this challenge.

Thanks in advance.