A flag is raised when the formula evaluates to true. This is often used to flag the task because of another task's input or selection.
The previous task asked if there was a sufficient amount of gas available considering the work scheduled for the next day.
The user selected No (there isn't a sufficient amount of gas). The checklist task was flagged, but now you (the configurator) also want the follow-up tasks to be flagged too, so that the necessary action plan can be implemented to resolve the issue.
Enter [previous_task_identifier]=False in the FlagX Formula column so they can be flagged too.