What the IDFV Is
The Identifier for Vendor is an ID iOS gives to apps through the identifierForVendor property of UIDevice. It is a UUID, a long alphanumeric value, and it has a specific scope: every app from the same vendor on the same device sees the same value, and apps from other vendors see different values.
"Vendor" is determined by the App Store developer account that publishes the app. A company with a main app and a companion app under one developer account gets the same IDFV in both on a given phone. A different company's app on that phone gets a different IDFV.
The IDFV exists so developers have a stable, permission-free way to recognise a device across their own apps for analytics, fraud prevention and account features, without a cross-app tracking identifier.
IDFV vs IDFA
The two identifiers serve different jobs. The IDFA, the Identifier for Advertisers, is the same in every app on a device and is meant for advertising. Since iOS 14.5, apps can read it only if the user grants App Tracking Transparency permission. Without permission, it returns zeros.
The IDFV needs no permission because it is limited to one developer's apps. Ad networks cannot see your IDFV, and your IDFV cannot be compared with values in other companies' apps, so it does not support cross-company ad attribution. Apple's guidelines also prohibit using it, or any identifier, to track users across other companies' apps and websites without ATT consent.
In short: the IDFA links devices across the ad ecosystem with permission; the IDFV links devices across your own apps without it.
When the IDFV Resets
The IDFV is stable while at least one app from your developer account remains on the device. If the user deletes all of your apps and later reinstalls one, iOS generates a new IDFV. That makes the IDFV a weak signal for reinstall detection if the user had only one of your apps. Erasing a device also changes it, and restoring a backup to a new device may produce a new value.
The value can also be unavailable briefly in some situations, such as right after a device restart before the user unlocks it, so code should handle a missing value.
How IDFV Is Used in Measurement
Inside your own data, the IDFV is a useful device key. It can deduplicate installs and sessions, link analytics events from the same device, and support fraud checks such as spotting many accounts on one device. Companies with several apps can measure cross-promotion between them.
For ad attribution, its role is limited. iOS install attribution relies on signals that ad networks can also see or that Apple provides: Apple AdServices for Apple Ads, ad click IDs where they survive the redirect, SKAdNetwork and AdAttributionKit postbacks, the IDFA when the user allows it, and probabilistic matching as a fallback. Linkrunner matches installs deterministically first using these signals, and falls back to IP and user-agent matching only when no deterministic signal exists. See SKAN and AdAttributionKit for how Linkrunner handles iOS privacy frameworks.
Privacy Considerations
Because the IDFV is a persistent identifier tied to a device, privacy laws such as GDPR and the CCPA can treat it as personal data. Include it in your privacy disclosures and App Store privacy labels, cover it in deletion requests, and do not share it with third parties in ways that amount to tracking. Using it only for first-party analytics, fraud prevention and account features keeps you within both Apple's rules and the spirit of most privacy laws.
IDFV in Your Data Stack
In practice, the IDFV often ends up as one of several keys in your data. Analytics tools use it as a device ID, your backend may store it against accounts, and your MMP may receive it with installs and events. Keep its role clear so it does not drift into uses Apple's rules forbid.
Use your own customer user ID as the main key for people, and the IDFV as a key for devices. A user with two iPhones has two IDFVs and one user ID. A shared family iPad has one IDFV and possibly several users. Mixing the two leads to double-counted users or merged profiles.
When sending data to partners, send the IDFV only where it serves a first-party purpose you can explain, such as deduplication, support or fraud checks. Do not use it to join your data with other companies' data about the same device. If a partner asks for the IDFV to match users across apps that are not yours, that is tracking under Apple's definition and needs ATT permission.