Every card transaction carries a terminal identification number, or TID: the code assigned to a single point of acceptance — one card machine, one lane, one online endpoint — so the acquirer can tell which device sent it. It sits alongside the merchant's own identifier in the message and never changes for the life of that terminal.
Where the number comes from and how it reaches the device
The TID is issued when a terminal is boarded, by the acquirer or by the processor running the estate. It is written into the device during its configuration download, alongside the trading name, the currencies it may take and the address of the acquiring host it should talk to.
From that point the value rides with every authorization request and stays attached through capture, clearing and reporting. Nothing else in the message says where the payment physically happened.
The settings that travel with a TID
A TID is not only a label, it is a profile. Tipping on or off, which currencies are offered, whether currency conversion is presented at the point of sale, contactless limits, receipt text, and whether the device may fall back to offline authorization when the network drops are all held against the terminal record.
That explains the most common support ticket in card acceptance: the same transaction works on one machine in a shop and fails on the one beside it. The machines are not identical, because their TID profiles are not.
When disputes cluster on one terminal
Terminal-level identity is what makes a problem findable. If declines, refunds or chargebacks concentrate on a single TID, the cause is local — one site, one shift, one badly configured integration — and the fix is to reconfigure or block that terminal.
The alternative, suspending everything the business runs, costs revenue across sites that were never involved. Diagnosis at TID level is the difference between a two-hour fix and a shutdown.
Terminal IDs for traffic with no terminal
Card-not-present volume uses the same field with no hardware behind it. The payment gateway assigns a value when a store, endpoint or channel is created, and an online retailer typically ends up with separate ones for its website, its app and its phone-order desk.
Names vary by provider: terminal ID, device ID, store number, source ID. The function does not change.
TID and MID answer different questions
A merchant identification number identifies the business relationship — the contract, the pricing, the account that receives money. A TID identifies the endpoint sitting under it. Money settles at MID level and diagnostics happen at TID level, which is why a restaurant group with five venues may run one MID per venue and six TIDs inside each.