Filtering by Discipline when running Reports in Bentley Substation

Good Day

I am busy working on customizing a report in Bentley Substation. Currently i have managed to customize the report to output all the parts used in a project, quantity and cost them. Now i am looking to further customize the report to break it down per discipline (i.e. structures, foundations, equipment etc.) - see below.

The problem that i am finding is that it i cannot apply a filter under the field column to select "Discipline". Why is this the case?

All comments or suggestions will be welcomed.