CRopeKeyframe

is a code class in every Source game. It defines a point on a rope.

Classnames:

FGD Code
For whatever reason,  and   have slightly different FGD info, which causes them to behave a little differently in Hammer. This forces mappers to start their ropes using  then create every other segment with , unless they're willing to tolerate some minor drawbacks.

Using only  causes the first segment visualization to not appear (this can be avoided by creating one extra rope point), and using only   is tedious because the user has to change two keyvalues for every rope point by hand as opposed to using +Drag.

Regardless of which route the mapper goes, the rope will work the same ingame. The FGD code below gets rid of the differences, which will allow you to use whichever one you want. (This has already been done for you if you're using the full SE FGDs.)