Rectangular marquee tool, ellipse marquee tool, columns, freehand, magnetic etc
Create a new one, you can use this mode to move it around as well as to just create and replacing an existing one. It uses the existing feather so will be sharp if 0 and blurry if greater than zero
Adds to the existing one. If you overlap, it will join with the original. If you apply it away from the original, it will still be created but not part of the original.
Will not work if there is no marching ants (you can remove them using the select menu and deselect). If you cross over / overlap an existing one then it will remove from that. Area that is not covered by the subtract will remain.
Has to have one existing. If you use this mode, drag over an existing and only that part covered by the tool will remain, the rest will be removed.
If you apply one with it set to 0 then it will be a sharp edged set of marching ants. If you then set it to 45px, the previous will be untouched, only the new modifications will be feathered / blurred.