Identifiers are attributes or mechanisms that are primarily used to establish the identity of a user. They are an important building block in much of performance marketing because they help tie different marketing touchpoints (such as ad exposures and paid search clicks) to actual success events (conversion events).
Identifiers come in two flavors: deterministic identifiers (which can be used to definitively identify a user or device) and probabilistic identifiers (which can be used to approximate the identity of a user or device). The most common types of identifiers are cookies, IFAs, PII and device fingerprints/snapshots
