Western Union has special tracking codes that it calls Money Transfer Control Numbers, or MTCN, which are essentially tracking numbers. Whenever a person uses the company for a money transfer service, it gives the sender this particular 10-digit code to keep as a reference.
From Western Union's website, senders can use the code, along with their first name and last name, to check on their order.
The recipients of transfers must have the MTCN to pick up their money from a Western Union location. It's a unique identifier for the transaction. Without it, the Western Union representative wouldn't be able to release the funds to the recipient.