set-mark

set-mark is a compiled Lisp function in `simple.el'.

(set-mark POS)

Set this buffer's mark to POS. Don't use this function!
That is to say, don't use this function unless you want
the user to see that the mark has moved, and you want the previous
mark position to be lost.

Normally, when a new mark is set, the old one should go on the stack.
This is why most applications should use `push-mark', not `set-mark'.

Novice Emacs Lisp programmers often try to use the mark for the wrong
purposes. The mark saves a location for the user's convenience.
Most editing commands should not alter the mark.
To remember a location for internal use in the Lisp program,
store it in a Lisp variable. Example:

(let ((beg (point))) (forward-line 1) (delete-region beg (point))).