2 in edit mode for the various contextual operations described below
SHIFT + 2 in edit mode to toggle Sharps or Chamfermodifier or Korean Bevelmodifier
CTRL + 2 in edit mode to offset edges (Korean Bevel on the mesh level)
Smart Edge simplies various common task around Edges.
in vert mode with one or less vertices selected: invoke Knife operator
in vert mode with more than 1 vert selected: run Path or Star connect
in edge mode with nothing selected: invoke Loop Cut operator
in edge mode with 1-3 edges selected: turn the edge(s)
in edge mode with 2 sets of manifold edges selected: run Bridge operator
in edge mode with 4+ edges selected: select either region(faces) from loop or select adjacent faces, based on operator properties
in edge mode with with edges selected, that are separated from the rest of the mesh: knife project
in face mode with nothing selected: invoke Loop Cut operator
in face mode with faces selected: select either bounds(egdes) from region or select adjacent edges, based on operator properties
in face mode with faces selected, that are separated from the rest of the mesh: knife project
Using Smart Edge
This is an old video and does not cover all the features described above.