Alias

An alias is an explicit instruction to an analytics tool that two identifiers belong to the same person, so their event histories are treated as one profile.

The call carries the old identifier and the new one; the tool writes a mapping row and from then on resolves both to a single canonical identifier. The exact semantics differ by tool, so the vendor's own documentation decides the behaviour: in Mixpanel the call is meant to fire once, in the sign-up direction, attaching a new identifier to an existing distinct ID, while in Segment it mostly exists to carry the anonymous-to-user mapping through to downstream destinations. It is not the same as identify, which attaches an identity and its traits going forward, whereas an alias asserts that two identities were always the same person. In most tools the operation cannot be reversed, and a repeated or wrongly directed call builds an identity chain that can weld two real people into one profile.