It puts vertices, and creates a line or a face.
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| Line | Triangle | Face | Polygon |
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 Cnfiguration.
If you click with Shift-key for 2nd or later vertex, a vertex moved only horizontally or vertically from the previous one will be put.
If you click on an existing face, a vertex will be put on the face.
FrontIt creates a front side of face.
BackIt creates a backside of face.
BothIt creates a both side of face.
Unlike a double-sided face in the material property, two faces are actually created.
LineIt creates a line when two points are put.
You can use [Menu > Object > Fill place surrounded by lines] to create faces at the place surrounded by lines.
TriangleIt creates a triangle when three points are put.
QuadrangleIt 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.
PolygonIt creates a polygon. Please click right button after three or more points are put, and a polygon will be created.
It adds a point at the specified X/Y/Z position along the specified world or local coordinate.
Property