I don’t know if this explanation will help, you may need to wait for Wayne or Adrian to answer, both considerably smarter than me. Anyway, the forearm_R has the IK constraint because in the rigging of rivet that is the bone that has a constraint on it, as for animating it you don’t care about the constraints. If you linked rivet in I don’t think you have the edit option, part of the point of linking. is to prevent you from making any changes, ie animate only. If mine has the edit option available I frequently appended and didn’t link in the character. I once in awhile edit the mesh, add bones, or just want to see how something works. So can you make the fk bone move now and are you just wondering about constraints?
There is this course on rigging, that explains ik, fk, bone constraints etc. https://cgcookie.com/course/fundamentals-of-rigging-in-blender-course.
