This filter works only when the option Allow respondents to save the submission and resume later is enabled (in Settings → Advanced). It refers to the column Status located in Submissions. You can use this column to filter submissions in your custom reports.
You can either filter submissions that are finished or incomplete. An incomplete submission means that the submitter has saved the submission to resume it later, but has not done it at the moment. Once they submit the form, the submission is finished.
In Reports, select your custom report and hit Edit Report.
Afterwards, click on Report Filters and hit Add Filter.
Select Submission status in the dropdown and select either of the conditions is finished or is incomplete, depending on your needs. Next, press Add Rule and that’s it.
You can filter reports based on submission status for the following report widgets: Chart, Numeric and Data Table.