Tracking user transactions with transactionID.
Web Service API
Each time there is a payment for a billing period, there are two transaction IDs generated. The OriginalTransactionID remains the same for all billing cycles, whereas the transactionID will be unique for each billing cycle.
The only time an OriginalTransactionID can change is if a user cancels their subscription and later re-subscribes. Upon resubscribing, he/she would be given a new OriginalTransaction ID (since they would be viewed as a new user).
If you have a user's transactionID, you can always find the OriginalTransactionID. You can also find the primary transaction ID using the getPurchases command on device.