The circle selector gives you a circular cursor to use in selecting vertices, edges, faces, and objects. The selection with the circle selector is cumulative. So, using it adds to the vertices that have been selected:
Press the A key to deselect everything.
Press the C key. Notice the circle around the cursor. Click theLMB over vertices to select them. You can also hold down theLMB as you move the mouse to select more vertices.
To stop the selection, either click theRMB or the Esc key.
Press the C key again. Hold down theMMB while moving over previously selected vertices.
You learned about using the C key to initiate the selection of vertices, by holding down theLMB and dragging the cursor, or clicking theLMB. You learned to press theRMB or the Esc key when you are done. Pressing theMMB instead of theLMB after pressing the C key deselects vertices.