Can anyone explain what "nubbin" is as used in ASME Section VIII, Div. 1, Appendix 2? Is there another text that provides good examples of a flange that has this component?
A nubbin is just a small tooth obtained by machining on the joint contact area of a flange. This is a very detailed issue and is normally specified by joint manufacturers for use with specific types of joints.
You should find more in a good catalog or site of a joint manufacturer.