'Flagged' and 'Unflagged' (sometimes called flocked and unflocked) are descriptions referring to the bristles of a brush. They have had the last 8mm or so of their bristles split into lots of finer parts, making one bristle seem like many very small ones. This makes for a very soft brush, ideal for gentle cleaning and areas where the windows are quite clean. Flagged brushes are often used when cleaning vehicles.

Unflagged (or monofilament) brushes have bristles which are plain all the way to the ends, making them a little more aggressive in their cleaning method. Although not stiff enough to damage windows and frames they are less gentle in use.
https://www.cleaningspot.co.uk/waterfed_pole_brushes