On a Jira Agile board, it is possible to "Flag" an issue.
This is achieved by selecting an issue on the board and in the Details section choose Action -> Add Flag.

This sets the custom field 'flagged' with the value 'impediment'.

If this field ("flagged") is not in the Field Configuration of the project, the field is not filled/set, but the flag does appear on the Agile Board.

This is very confusing, because if you create a filter for this attribute, it will always show zero issues. (JQL: "Flagged = 'impediment'" or "Flagged is not empty")

It would be better if the Add Flag option is disabled if the Field is not present on the Field Configuration