whenever following events occures
" S1 intra system Handover Triggered", or
"S1 inter system Handover Triggered" or
"X2 Handover Triggered"
if eNB decided its not sure that UE specific NAS message what it received from MME will be delivered, so in that case eNB will send back to MME .
When MME receives "NAS non delivery indication" from eNB ,MME shall retransmits the PDU or or shall be discarded based on NAS messages
for example :
TRACKING AREA UPDATE ACCEPT
SECURITY MODE COMMAND etc. may take following action,
**upon successful completion of the handover the MME shall retransmit the NAS PDU.
if handover failed and if NAS signaling exists MME shall retransmit .
and for some NAS messages like DOWNLINK NAS TRANSPORT etc MME will discard even handover is successfull.
ref: 3GPP TS 24.301
this is my understanding please let me know if i am wrong