Cave divers use a distinct set of markers to determine the direction and distance travelled in a cave. These markers must be identifiable through sight and touch should the divers find themselves in limited or zero visibility. Many wreck divers have also adopted using these directional and non-directional markers.
Directional markers are known as "line arrows" in the cave diving community. Many popular cave and wreck diving systems have permanent line arrows at regular intervals (for example, every 30 metres or 100 feet) pointing to the nearest exit. If there is a numerical value displayed on the line arrow, it is most likely identifying the distance to the nearest exit.
Non-directional markers are known as "cookies" in the cave diving community. Cookies are used to mark reference points for divers during circuits and traverses, distinguishing lines at a 'T' intersection, or placed by each member of a team on a jump or gap line to identify who has exited should the team get separated. Non-directional markers should have some form of personal identification printed on them such as initials, or drawings. Some cave divers add a personal tactile element to help identify their marker should they encounter zero visibility; these tactile identifiers may include slits on the side or holes punched through the marker. A non-directional marker is only to be referenced by the diver or team who installed it and should be ignored by others.
Referencing Exit Markers (REM) are a relatively new marker designed to be used as an arrow by the diver placing it and a cookie by others. It's a hybrid marker. A REM is rectangular in shape with slots included enabling divers to attach it to a line. It also has blank space available to include personal or team identification on one side and a small slate on the other to write on for reasons such as survey work, team separation (i.e. I exited the cave at 37 minutes), and more.
You can trust the range of top-quality line markers and cookies from The Scuba Doctor.