Real estate terms listed in online real estate dictionary.
"A parcel of land which is typically located behind another lot facing a main street that is in the shape of a flag on a pole which provides access to the site."
flag land real estate term - Google Search
I had not heard the term before you just asked, but the above sound good to me.
I assume they mean something like this...
Polk/Des Moines Assessor - 090/02901-001-000 Listing