The udev rules included into dmsetup are responsible for setting up the devices
in /dev/mapper and also provides some environment variables for use by later
rules:

* DM_NAME
  Actual DM device's name
* DM_UUID
  UUID set for DM device (blank if not specified)
* DM_SUSPENDED
  Suspended state of DM device (0 or 1)

Other device mapper using packages may define there own rules. For example lvm2,
see /usr/share/doc/lvm2/README.udev for further informations.