-If used without any arguments, C<virt-df> checks with libvirt to get a
-list of all active and inactive guests, and performs a C<df>-type
-operation on each one in turn, printing out the results.
+If any I<-a> or I<-d> arguments are specified, C<virt-df> performs a
+C<df>-type operation on either the single named libvirt domain, or on
+the disk image(s) listed on the command line (which must all belong to
+a single VM). In this mode (with arguments), C<virt-df> will I<only
+work for a single guest>. If you want to run on multiple guests, then
+you have to invoke C<virt-df> multiple times.