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OSM Notes is a core feature of the OpenStreetMap.org website for placing shared notes on the map to assist in mapping/editing OpenStreetMap. The notes allow both registered users and the wider public to quickly indicate where there are errors or omissions in the OpenStreetMap data, and for contributors to OpenStreetMap to know where their support is needed.
Actually it is not necessary, the function works above any map data, but if you are willing to edit OSM maps, it is understandable. It is also possible to open another map in overlay mode to compare and for better placement of your new notes.
Start OSM Notes function from Menu > More functions > Notes (OpenStreetMap). The function's dialog window is divided into a few sections:
It may require zooming your map out to see any notes as there are usually not many of them. The data are loaded from a server when you stop moving the map so it may take a while till they are displayed.