One plus 3T - Manual (Page 6)

6 | What’s in the Box Device OxygenOS Safety Information
Alert Slider
The Alert Slider allows you to effortlessly switch between
3 notification profiles without having to take your phone out of your
pocket. With the OnePlus 3, switching between alert modes faster
and easier than ever before.
None
Priority
All
Disables all notifications
(alarms will still be enabled)
Pushes notifications only from your
priority apps and contacts
(which you can program in Settings >
Sound & notification > App notifications)
Allows notifications from all apps
and contacts.