Originally Posted by Metaluna

Here's a good explanation of all the Jagwire end cap types

Jagwire End Caps Guide | Jagwire

The "spike" is actually a tube that is supposed to protect coated cables from rubbing against rough edges in conventional ferrules and housing stops, and also provide a degree of dirt and moisture protection.

The "POP" ferrules (aka end caps) with the step-down diameter are designed to fit into housing stops (such as in some brake levers) that were designed to accept bare spiral housing without a ferrule, and thus are too narrow to accept the Jagwire end caps. Jagwire's compressionless housing needs special end caps to terminate it properly, so just sticking the bare housing into a lever body that was designed for spiral can cause problems, apparently. I even saw a YouTube brake setup tutorial recently where someone slid a conventional ferrule over the narrow section of the POP ferrule, I guess to either make it look better or to shim it out a bit.