ne's manual

4.4.1: Mark

The mark can then be used in order to perform clip operations. The clip commands act on the characters laying between the mark and the cursor.

If you call this command with no arguments, it will toggle the mark. If you specify 0 or 1, the mark will be canceled or set to the current position, respectively. A capital `M' appears on the status bar, if the mark is active.

See Record, for the reason why the mark is implemented as a flag.