# External ID

> An external alias for a Client ID — an email, phone, or your own customer ID — used when the Click ID can't reach a destination such as a CRM.

The External ID (`refId`) is an external alias for a Client ID — an email, phone number, or your own customer ID — that you supply when the Click ID cannot reach a destination such as a CRM.

## When to use an External ID

A Click ID travels through links, forms, and integrations. When it can't (for example, a contact created directly in a CRM, or a destination that only stores your own customer record), you alias the visitor with a `refId`. Later events identified by that value match back to the same visitor.

## External ID vs Client ID

The Client ID is the anonymous cookie AnyTrack sets. The External ID is a value *you* control and supply. AnyTrack matches a returning conversion back to the original session by the Click ID first, and by the `refId` when the Click ID could not travel.