In a report I have a calculated control which is the difference between two numbers. Is there any way that I can sort using that control? When I refer to it in VBA in an orderby it asks for the parameter of that control since it is not an actual field. Neither can I sort by it in the sorting & grouping options again because it is not an actual field?

You need to create the calculated field in your report's recordsource query. Then you can use the sort/group options of the report to sort by it.

Select ..., Nz(FldA,0) + Nz(FldB,0) As CalcFld
From ...;

Thanx but I don't think I can do that because my control is actually the difference between a field on the report and a field on a subreport which is linked to the report. If I would be able to refer to both in one query I would do away with the subreport but I can't since sometimes the result of the query for the subreport is null and that would make the source for the main report null aswell

If you can't calculate it in the query, you can't sort by it since you can ONLY sort by fields in the report's recordsource. It's as simple as that.