see
http://wiki.openstreetmap.org/wiki/Proposed_features/Terrain_accident

and contour layer comments, and the CycleMap mapnik slippy tiles
http://wiki.openstreetmap.org/wiki/Contours#The_PostGIS_approach
http://www.openstreetmap.org/?lat=-43.9989&lon=-176.5611&zoom=13&layers=00B0FTF
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RP: a type of contour, hence not included in the main DB
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HB: seems like (*I haven't actually looked) the edge of a crater or an escarpment, i.e. a physical ground feature, as opposed to a contour line which is arbitrarily chosen at some interval as a reflection of the elevation. Thus the edge of the depression is chosen by nature, while the placement of the contour line was chosen by a human.