Soheil Bakhshi explains the concept of report-level measures:
Report level measures are the measures created by the report writers within a Thin Report. Hence, the report level measures are available within the hosting Thin Report only which means the report level measures are not written back to the underlying dataset and hence they are not available to any other reports.
Read on to see how you can create one. I don’t think I’d ever heard them called that before but the name makes sense.