udev man manual

      udevd  allows the serialization of hotplug(8) events. The events gener-
      ated by the kernel may arrive in random order in userspace, that  makes
      it neccessary to reorder them.
      udevd  takes  care  of  the  kernel supplied SEQNUM sequence number and
       arranges the events  for  execution  in  the  correct  order.   Missing
       sequences  delay  the execution of the following events until a timeout
       is reached. Events without any sequence number are bypassing the  queue
       and will be executed immediately.
      For  each  event  a udev(8) instance is executed in the background with
      the complete environment received by the  hotplug  event.  All  further
      events for the same device are delayed until the execution is finished.
      This way there will never be more than one instance running for a  sin-
      gle device at the same time.