R - Commands   
RAS (command)
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It highlights the movement of the cursor on lines and vertexes of the background Raster files, if existing.

 Snap toolbar: 

  Menu : ToolsRaster Snap

 Command line: RAS

 

It Enables/Disables snap on lines and vertexes (drawn in a dark color on a light background) of the background Raster file, if existing.
Raster vectorizing is in this way made easier (drawing precise lines on it).