If you click a left button with Ctrl-key, the new face shares a vertex contained
by the existing face. If you don't push Ctrl-key, it's influenced by 'Distance to
select' in the configuration.
Front
It creates a front side of face.
Back
It creates a back side of face.
Both
It creates a both side of face. Actually two faces are created.
Line
It creates a line when two points are put.
Tri (Triangle)
It creates a triangle when three points are put.
Face
It creates a quadrangle when four points are put. And it creates a
line or a triangle when two or three points are put and click the right
button.