Hi Praveen,
I think they should be in sequence as that is what i have seen so far in the messages received but i have asked for the party to confirm. However, before the 3rd party comes back with message with segments not in sequence, i want to be ready with the solution which should be confirmed latest by tomorrow. I will test the UDF also before the end of today. Thank you again for this. 
Another issue that I am facing is that for the field Qty, if it is not received or the tag is empty, then i have to map with default. However, with this i tried various permutations, but i am not getting anywhere.
Any ideas on this?
Regards,
Archana