Crimping slot

Crimps are metal sleeves used to make a loop in heavy line without tying a knot, which matters with wire leaders and with monofilament heavy enough that knots become unreliable. The slot in the pliers is shaped so it compresses the sleeve evenly around the line rather than flattening it.

In freshwater it comes up mostly for pike and musky leaders. Most anglers will never use it.

The same slots get used informally for pinching split shot, which they do neatly, and for crimping barbs flat.

Most pliers carry two or three slots in different sizes, because a sleeve has to match the line diameter to grip properly. Using the wrong slot either fails to close the sleeve or cuts into it, and a cut sleeve is a connection that will fail under load.

Double-sleeve crimps, where you run the tag end back through a second sleeve, are the standard way to make this reliable on heavy leader.