The digitizer allows editing of 2D vector features (points, lines, centroids, boundaries, and areas). Vector features can be selected by mouse (bounding box or simply by mouse click, see select threshold in Settings→General→Select threshold), or by query (eg. by line length, see Settings→Query Tool). Dead (deleted) features are only marked as ‘dead’ in the geometry file but remain there and occupy space. Any vector module used afterwards on this vector map which really reads and writes vector geometry (so not g.copy) will write only features which are ‘alive’. Added or modified vector features are snapped to existing vector features (Settings→General→Snapping). To disable snapping set the snapping threshold to ‘0’. If the digitizer crashes for some reason, the changes are automatically saved. Broken topology can be repaired by running v.build. GRASS GIS uses a topological vector format, meaning that a common boundary of two polygons is only stored once. When digitizing polygons it is thus important to be able to only draw each boundary once. When drawing a polygon adjacent to an existing polygon, one has to first split the existing boundary at the points where the new boundary will be attached. Snapping should be set to ensure that the new boundaries are automatically attached to the chosen points. Source.