<Feature Face Ring> and <Feature Face>
A <Feature Face Ring> is used to specify a boundary of a <Feature Face>, and is specified as a sequence of <Feature Edge> instances laid end to end.
A regular <Feature Face> instance (universal = SE_FALSE) treats its first <Feature Face Ring> as its external boundary. All other <Feature Face Ring> components are internal boundaries, representing "holes" in the <Feature Face>.
Detail, Class Diagram Sheet 12