Witness marks are used to mark the alignment of optics to give you a quick visual reference of where they go back on your weapon if removed. This is particularly handy if you’re using a quick-disconnect mount, such as a LaRue mount, on a rail system.
Without getting into zeroing your weapon too much, these marks will make it easier to re-zero if you have to take your optics off for any reason. LaRue mounts are great, but you should always recheck your zero if you have the time and opportunity to after removing and remounting an optic.
Witness marks are also handy for marking your screws/bolts on your weapon to give a visual reference of any screw-loosening shenanigans your weapon might be playing with you.
We favor DecoColor paint markers here at ITS because they seem to last better than other brands, and you can get them in various tip sizes.