When selecting a cylindrical face that happens to be part of a machining feature in order to define a clipping on its axis, then this only works if the feature consists of only one face. If it consists of more than one face then it produces the error message "Single selection required".
Workaround: change the selection focus to "Face" and then reselect the face.
It should not be necessary to have to change the focus. It should be made possible to set the standard selection to "Face" rather than "Custom Feature". It would still be possible to set the focus to "Custom Feature" if required.
