The IDFV (Identifier for Vendors) is an Apple device identifier that is the same across every app from a single vendor (developer) on a given device, and different for other vendors. It is the counterpart to the [[idfa]] (Identifier for Advertisers), but built for a different purpose: first-party measurement within one company's portfolio rather than cross-publisher ad tracking.
The key difference is consent. [[idfa]] requires an [[att]] (App Tracking Transparency) prompt and is only available if the user opts in — which most don't. IDFV needs no prompt and is available by default, precisely because it can't follow a user across different companies' apps: its scope is limited to one vendor. That makes it useful for stitching sessions across your own family of apps, deduplicating users, frequency capping within your portfolio, and some fraud signals — but useless as an advertising identifier across publishers.
Reset behavior: the IDFV persists as long as the user has at least one app from that vendor installed. Delete all of a vendor's apps and the IDFV is discarded; install one again and a new IDFV is generated. Android has no exact equivalent — the closest, the [[gaid]], is a cross-publisher advertising ID more analogous to IDFA.