DM web UI generates reports based on the current DM context: This could be a snapshot, a workspace or a system (composite) context. It all depends on VVC. You cannot create a report on multiple contexts.. Reports are document-style and not metrics reports.
You must save the current parameters that are specified in the report. You can choose the format that is supported by the RPE engine underneath. Depending on the format you select it opens a temporary window opens while the report runs; you are prompted to save or open the generated report in the selected format.