The first argument is a combination of two flags. The first flag, TPM_CENTERALIGN, centers the pop-up menu on the mouse point. You can also use TPM_LEFTALIGN or TPM_RIGHTALIGN instead. These flags line up the left or right edge of the pop-up menu with the mouse position. The second part of this flag combination is TPM_LEFTBUTTON, which makes the pop-up menu trigger from the left mouse button. You can also use TPM_RIGHTBUTTON to make the menu trigger from the right mouse button.