Unique Identifier
A Unique Identifier is used to guarantee a profile is unique. A profile can have several identifying values — different profiles can have the same identifying value (the same email address, for example) — however, never within the same Unique Identifier type.
🔑 The key idea
A profile can have several identifying values (multiple emails, multiple phone numbers), but only one of them is its Unique Identifier. Different profiles can share the same identifying value — but never within the same Unique Identifier type.
In this article
What is a Unique Identifier?
The Unique Identifier tells Apsis One: "This combination of identifier type + value points to one specific profile, and only one." Apsis One supports three Unique Identifier types out of the box:
Email
Mobile Number
CRM ID
Here's an example of two different profiles with the same email address but different Unique Identifiers:
Profile | Unique Identifier | Email address | First name | CRM ID |
1 | Anna | — | ||
2 | CRM ID | Anna | 1234 |
Both profiles can co-exist because they're identified differently. Profile 1 is the email address itself; Profile 2 is the CRM record (which happens to also store that email).
Keys and Keyspaces
Behind the scenes, a Unique Identifier is a Keyspace. Profile Keys are the string values that identify a profile within that Keyspace. A profile can have keys in different Keyspaces, and several keys in the same Keyspace.
A profile is identified by a Keyspace combined with a Key. Every combination of Keyspace + Key points to either 0 or 1 profile.
How to choose a Unique Identifier
Your choice depends on your business, data structure, and goals. Most accounts use email, but each option has trade-offs.
Option | Best for | Watch out for |
Email-led communication, where one email = one person. | Profiles can only have one email as Unique Identifier. | |
Mobile | SMS-led businesses where the phone number is the primary touchpoint. | Same constraint as email — one mobile per profile as identifier. |
CRM ID | Households, shared inboxes, or B2B accounts where multiple people share an email. | Email and phone can no longer be used to search; use the CRM ID instead. |
Email as Unique Identifier
Apsis One sets email as the default Unique Identifier (the Email Attribute equals the Email key in Keyspace). This is the right choice for most senders, since an email address is usually unique to one person.
Changing the email address
For profiles in only one Section, the Profile key updates along with the new email address.
Profiles in multiple Sections keep the original email address as Unique Identifier.
Mobile Number as Unique Identifier
The same principle applies as with email — one mobile per profile as the Unique Identifier, with multiple mobiles allowed as Attributes.
CRM ID as Unique Identifier
A profile with a CRM ID as Unique Identifier can share an email address or phone number with another profile. This is exactly what you want when:
A married couple shares an email address.
Multiple people in a B2B account share a generic inbox.
You're syncing from a CRM where the CRM record, not the email, is the source of truth.
Each profile keeps its own personalisation, behaviour data, and click tracking — so two recipients on the same email address still get two individual experiences.
How does it work in practice?
Imagine a married couple sharing one inbox. Both opt in to your communications and create separate profiles in your CRM (and therefore in Apsis One). When you send a campaign with Data Tags or Row Segmentation, both profiles receive a personalised email addressed to them, with content matching their individual preferences. One email address, two recipients, two different experiences.
📝 Note on CRM IDs and search
When CRM ID is the Unique Identifier, email and phone number used to search will return partial matches and you need to identify the correct profile within the search results. Use the CRM ID belonging to the profile to search instead.
Switch Unique Identifier
You can switch a profile's Unique Identifier — for example, moving James from being identified by email to being identified by CRM ID — without losing any of his historical event data, consent, website interactions, or response data.
Profiles consist of a Keyspace and a Profile Key. Any profile can have multiple Profile Keys, but each must belong to a different Keyspace. Switching Unique Identifier removes one Profile Key and replaces it with a new one in a different Keyspace.
💡 What stays the same
Changes in Unique Identifier do not impact marketing activities. Profiles continue entering and proceeding in Marketing Automation flows as normal as long as they still meet the criteria for the flows.
Switch with the API (single profile)
For single-profile changes, use the API's Switch Unique Identifier endpoint. See API documentation →
Use the Apsis One API (only single Profile)
Watch this video to learn how to Change the Unique Identifier for a single Profile with API.