A number of newer Fibit wearables were shipped with SpO2 (or estimated blood oxygen) sensors, which may not have been activated within their systems. However, the OEM has addressed this in a new update. Therefore, compatible devices can now track this additional health metric for their users.
https://www.notebookcheck.net/A-new-Fitbit-software-update-unlocks-blood-oxygen-tracking-on-several-devices.450319.0.html