Reference
Prefixed object IDs
API v2 objects now use prefixed IDs, such as em_ for emails and dom_ for domains, so you can tell an object's type from its ID.
Updated Jan 27, 2026
ChangedREST API
IDs are unique across services and sort by creation time. Common prefixes are em_ for emails, dom_ for domains, key_ for API keys, aud_ for audiences, sub_ for subscribers, con_ for contacts, sup_ for suppressions, wh_ for webhooks, evt_ for events and cmp_ for campaigns.
Store IDs as opaque strings rather than parsing them.