Meaning of Status information in deliver_sm

Dear Kannel Devel,

The status info in the deliver_sm can be one of the following:

DELIVRD
  EXPIRED
  DELETED
  UNDELIV
  ACCEPTD
  UNKNOWN
  REJECTD

EXPIRED is the SMSC tried to send the SMS but the ESME was down for the
period of trying to send the SMS
DELETED as far as I understand it's when you issue a command to delete the
SMS before sending it.

Questions:
1)DELIVRD means the SMS arrived to the ESME, what does ACCEPTD mean?
2)If UNKNOWN means the msisdn was not found in the HLR what does REJECTD
mean?
3)When do you get a UNDELIV rather than one of the other error conditions
such as: UNKNOWN
  REJECTD, or EXPIRED.

If anyone has experience interpreting this, I'd appreciate if they will
share their knowledge.

Thanks