I think you can add multiple , like netflowReceiver.0.ip=X.X.X.X
netflowReceiver.1.ip=Y.Y.Y.Y
As each of these instances has fields for IP, port etc , I think its purpose it to be able to add multiple.
Never tried it 
I only have 1 Mikrotik
These sections on
netflowElement.1.id = 226
netflowElement.1.termid = netflow.postNATDestinationIPAddress
etc..
I think I added them while playing to get IPFIX working (IPFIX works with periodic templates of fields being transmitted).
I believe the default “dictionary” did not recognise certain Netflow field, hence the possibility to add new ones.
To be honest I don’t know anymore. They are not present in the “default” files so for sure some custom work.
Perhaps it something to do with this thread NetFlow. No longer showing NAT'd destination address - Something chnaged