Why is the number of users in my Segment lower than the number shown in Tag Statistics?

It is common to see a discrepancy between the total number of devices associated with a Tag and the estimated reach of a Segment. This usually happens for the following reasons:

1. Active vs. Inactive Devices

• Tag Statistics: These numbers represent all devices (HWIDs) that have the specific tag assigned, regardless of their current status. This count includes inactive devices, such as those where the user has uninstalled the application or where the push token has become invalid.
• Segment Calculation: When calculating a segment's size, the system counts only active (reachable) devices that are capable of receiving messages at that moment.

Example: If 25,000 devices have the tag NewsletterSubscriber, but 10,000 of those users have uninstalled the app, the Tag Statistic will show 25,000, but the Segment size will show 15,000.

2. Incorrect Channel Registration (Email/SMS)

If you are seeing a low number of users for Email or SMS channels specifically, verify how the data is stored: 

* The Pitfall: Simply storing an email address as a value within a Custom Tag (e.g., using the /setTags API method) does not register that email as a delivery channel. 
* The Solution: To send emails, you must register the user using the dedicated registerEmail endpoint. A segment filtering for the "Email" platform will only count users correctly registered via this endpoint, regardless of any custom tags they might have.

3. Cross-Platform Mapping

If a tag is assigned to a user's Mobile device profile, but you are building a Segment for the Email platform, the user will only be included if their Mobile and Email profiles are linked via a common UserId. If the profiles are not linked, the tag on the mobile device will not qualify the email address for the segment.