A UID, short for unique identifier, is a value that belongs to exactly one item and no other. In asset tracking the term is used in two related ways. In NFC and HF RFID, the UID is the serial number the chip manufacturer writes into every tag. In UID marking (often called "UID tagging"), it is a permanent machine-readable mark on a part or piece of equipment, best known from the US Department of Defense Item Unique Identification (IUID) program.
Both serve the same purpose: one scan or tap resolves to one specific item, not just a product type. That is the difference between a UID and a retail barcode, where every identical product carries the same code.
UID on NFC and HF RFID tags
Every NFC and HF RFID chip carries a UID set at the factory. Many common NFC chips, such as the NTAG family, use a 7-byte UID, shown as 14 hexadecimal characters (for example 04:A2:3B:1C:5D:80:80). It is the first thing a phone reads when it taps a tag. Because you do not need to write anything to the tag, linking the UID to an asset once is enough: every later tap opens that asset's record. The UID is the HF/NFC counterpart of the UHF TID.
UID marking and IUID (defense and government)
Many searches for "UID tagging", "UID labels" or "UID devices" refer to UID marking under the US DoD IUID program. Here a Unique Item Identifier (UII) is encoded in a 2D Data Matrix (ECC 200) symbol and applied to the item as a durable label, plate or direct part mark, following the MIL-STD-130 marking standard. The UII is built from an enterprise identifier (often a CAGE code), plus a serial number and, in one construct, the original part number. Contracts that include the relevant DFARS clause require marking of qualifying items, for example items above a unit-cost threshold or that are serially managed. Check the current clause and your contract for the exact rules.
| Item | NFC / HF chip UID | UID marking (IUID / UII) |
|---|---|---|
| What it is | Factory serial number stored in the chip | Permanent 2D Data Matrix mark encoding a UII |
| Who sets it | The chip manufacturer | The manufacturer or owner of the item, under the program rules |
| How it is read | Tap with an NFC phone or HF reader | Scan with a 2D imager or camera |
| Typical length | Often 7 bytes (14 hex characters) | Enterprise ID + serial (and part number in one construct) |
| Typical use | Tap-to-identify assets, access cards | Defense and government equipment, parts traceability |
UID vs serial number vs asset tag number
- Manufacturer serial number: printed by the maker of the equipment; useful, but formats vary and some items have none.
- Asset tag number: assigned by your organization, for example AST-004217, and printed on the asset tag.
- Chip UID: fixed in the NFC chip; you cannot choose it, but it never needs encoding.
A good register stores all three against the asset, so any of them can be used to find the right record.
Common mistakes
- Treating a product barcode as a UID. An EAN or UPC code identifies a product model, not one item.
- Relying on 4-byte UIDs for security. Use 7-byte chips, or add signature checks, where cloning matters.
- Assuming UID marking and NFC are the same. IUID marks are 2D codes read by a camera; an NFC UID lives inside a chip.
- Not storing the UID in the register. If the link is only in someone's spreadsheet, the tag is useless when they leave.
UIDs in Asetavo
When you link an NFC tag to an asset in Asetavo, the tag's identifier is stored against that asset, so a later tap opens the right record. NFC tagging is available from the Growth plan; QR code and barcode labels work on every plan, and you can generate unique asset numbers with the free asset ID generator. Asetavo is not an IUID registry or a MIL-STD-130 marking system; for defense contracts, follow your contract's marking and registry requirements.
