snap-always-switch ("sna")

See this command in the command table.

To access this command:

Digitize ribbon >> Snap >> Snap Mode >> Always Snap.
Using the command line, enter "snap-always-switch"
Use the quick key combination "sna".
Display the Find Command screen, locate snap-always-switch and click Run.

Command Overview

Toggles the mechanism for snapping to data.

There are two possible states, which interchange with subsequent uses of the command:

Snapping will be performed using any mouse button.
Snapping will only be performed with mouse button 3 (default).

Note: There's no need to set up snapping modes when digitizing data (say, a polygon feature) onto a wireframe map. The closest wireframe vertex will always be the point at which a line vertex is positioned.