Yes, we use these to help mark a line when direction may be a bit fuzzy. They should always be placed pointing out. They are both a visual & tactile (0 vis) reference. In most US caves, they mark every 100 ft.

Also, yes they typically come in most any color you can imagine, including glow in the dark. Most of mine are white & yellow.

I have a few on what looks like an oversized diaper pin on my wreck reel.

They are a very important part of a cave diver’s kit. They are usually already positioned in a commonly dived cave but you should still have them on hand in your BCD or reel just in case.

absolutely you should have them, when you leave the gold line you attach your jump line and put an arrow pointing the way out at the tie point.