-is a gauge value. The number of values and the data-types have to match the
-type specified in the identifier (see L<types.db(5)> for details). The time is
-specified as epoch (i.E<nbsp>e., standard UNIX time) or as a literal C<N>
-which will be interpreted as now.
+is a gauge value. A literal C<U> is interpreted as an undefined gauge value.
+The number of values and the data-types have to match the type specified in
+the identifier (see L<types.db(5)> for details). The time is specified as
+epoch (i.E<nbsp>e., standard UNIX time) or as a literal C<N> which will be
+interpreted as now.