# Unique identifiers

## Unique identifiers in the OS NGD

### OSID

The OS NGD primarily uses an identifier called the OSID (OS Identifier). OSIDs uniquely identify records within a feature type and have a maintained lifecycle, enabling them to support change tracking and linkages across datasets.

It should be noted that while an OSID is unique within a feature type, it may be shared across feature types when they represent the same real-world feature. For example, a point feature in the OS NGD Geographical Names Theme representing a water body will share the same OSID as the water body feature in the OS NGD Water Theme.

### UPRN, USRN, UDPRN and postcode

Other unique identifiers are also present in the OS NGD, including:

* UPRN (Unique Property Reference Number) for Local Authority Address records.
* USRN (Unique Street Reference Number) for Local Authority Street records.
* UDPRN (Unique Delivery Point Reference Number) for Royal Mail delivery points (for example, a mailbox or business unit).
* :new: Postcode is the identifier assigned by Royal Mail for the purposes of mail delivery. It's is the primary unique identifier for [Postcode Unit Point](https://docs.os.uk/osngd/data-structure/administrative-and-statistical-units/gb-postcodes/postcode-unit-point) (Great Britain) and [Postcode Unit Point](https://docs.os.uk/osngd/data-structure/administrative-and-statistical-units/ni-postcodes/postcode-unit-point) (Northern Ireland) Feature Types. Postcode is an important identifier for the [Postcode Unit Area](https://docs.os.uk/osngd/data-structure/administrative-and-statistical-units/gb-postcodes/postcode-unit-area) Feature Type, but as the features represent postcode parts (not whole postcodes), their primary unique identifier is Feature ID.

These unique identifiers remain the authoritative identifiers for their respective domains.

### :new: Feature ID

For a small number of feature types in OS NGD where it is not possible or necessary to have identifiers that persist over time, the Feature ID is used as the primary unique identifier. The [Postcode Unit Area](https://docs.os.uk/osngd/data-structure/administrative-and-statistical-units/gb-postcodes/postcode-unit-area), [Retail Area Aggregated](https://docs.os.uk/osngd/data-structure/administrative-and-statistical-units/functional-areas/retail-area-aggregated), [Retail Area Major](https://docs.os.uk/osngd/data-structure/administrative-and-statistical-units/functional-areas/retail-area-major) and [Retail Area Minor](https://docs.os.uk/osngd/data-structure/administrative-and-statistical-units/functional-areas/retail-area-minor) Feature Types use this unique identifier. The notional extents of the retail area features and Postcode Unit Area features are regenerated periodically (for example, monthly for the latter), at which point all features will get a new Feature ID.

## Other identifiers provided in the OS NGD

### TOID

The Topographic Identifier (TOID) is also present in the OS NGD, primarily to support customer migration from OS MasterMap products to the OS NGD. This is an optional attribute and should not be relied upon to complete data linking or for implementing Change-Only Updates (COUs). The reason for this identifier being optional is the improved currency on which the OS NGD is published, meaning the TOID, which is created in our existing OS product systems, will not always be allocated to features as frequently as they will be accessible via the OS NGD.