The transaction number is used in online banking as the secondary means of identification after the PIN. While the PIN (almost) always stays the same, and therefore can be reused, the TAN is only valid for one financial transaction. You have to use a new TAN for each online banking transaction.

A unique transaction ID generated by the merchant. This value will be returned by the IPF system to the merchant after the registration process was terminated. It serves as unique transaction identifier.

A unique number, in the range 1 to 9999999, allocated to a claim transaction prior to the transaction being transmitted from a GP Practice.