Network transaction ID
The reference the card network assigns to each authorization, which later transactions cite to show they belong together.
Visa calls it the Transaction Identifier, Mastercard the Trace ID. It comes back in the authorization response and is the only thing that ties a chain of transactions together in the network's eyes. The monthly subscription charge points to the first, authenticated charge. The hotel's increment points to the hold. The reversal points to whatever it reverses.
The reference decides whether a charge without the customer present gets approved. A merchant-initiated payment that can show its original, strongly authenticated transaction is exempt from SCA. One that can't looks like an online purchase with no customer and is often declined. Merchants switching payment provider sometimes discover the IDs didn't come along, and that their entire subscriber base has to authenticate again.