A flag is located on a row within the details table at the bottom of the DoView screen. Clicking on the flag circle simply flags the row for your future reference. You may do this to remind yourself that you have not yet finished putting information into the row or for any other purpose.

 

In the screenshot below the row associated with Step B has been flagged by clicking on the small circle in the Flag column. The fact that this row is flagged is shown by the block of orange color over the row ID at the left end of the row.