.B LABEL
device label or serial number, like USB serial number, SCSI UUID or
file system label. Up to 5 different sysfs files can be checked, with
-all of the values being required in order to match the rule.
+all of the values being required in order to match the rule. The
+.BR BUS
+key is optional, but if specified, the sysfs device bus must be able to be
+detemined by a "device" symlink.
.br
.RB "keys: " BUS ", " SYSFS_
.TP