C1 and C0 define how P0 and P1 are connected to some central point of a weld. For Glue, we’ve got that, and also these F values. The F values indicate points that are offset by some Vector3 from that central point (in that point’s object-space coordinates). For each of the F values, imagine that the Glue has two handles, one welded to P0 and one welded to P1, and both offset from those parts as defined by C_ and the F_ value. The Glue attempts to keep these two handles together.
If you want a rigid Glue, you’d use these four points to define coplanar points. However, you can also be a bit more creative and make different types of joints with Glue. If all the F values are the same, you’ve got a ball and socket joint about that point. If the values are collinear, you’ve got a hinge along that line.
I hope this sort of helps.
Edit: Not entirely sure, but I may be missing something. The wiki does mention that no F value should ever use the Z axis, and I’ve got no idea what that’s about. There may be something about all the points needing to be coplanar or something. However, so far I’ve been able to use Glue by thinking about them in this way. It may take some fiddling and experimentation, but I’m pretty sure this is at least the general idea.