All selected vertices are moved to their mirror image position.
The mirror plane passes through the 3D cursor at right angles to the active window.
Vertical line - Mirrors the selected vertices through a vertical line passing through the position of the 3D Cursor in the active Working-Volume view.
Horizontal line - Mirrors the selected vertices through a horizontal line passing through the position of the 3D Cursor in the active Working-Volume view.
Copy, Reflect, and Weld - Makes a copy of the selected vertices, mirrors them, and welds the indicated vertex of the original selected vertices to the mirrored indicated vertex. Welding will not occur if the 3D Cursor is not indicating a vertex before the Mirror action is executed.