From iOS 15, Apple has introduced the interruption-level field for iOS push notification payloads. It is designed to allow iOS devices to properly display notifications with respect to the user's Focus mode and Notification Summary settings. There are four possible interruption level values:

Interruption Level	Payload Value	Description
Passive	passive	Minimal interruption, used for information with no urgency attached.
Active (Default)	active	Similar to push functionality in previous iOS versions, used for information that the user may wish to consume as it arrives.
Time Sensitive	time-sensitive	Requires an app entitlement. Appears immediately, breaking through Focus mode and Notification Summary settings (if permitted by user). Used for information that requires immediate attention.
Critical	critical	Requires an approved entitlement. Appears immediately, overriding ring/silent settings. Used for health and public safety updates.

Apple has a guide to interruption levels, the situations in which they should be used for and how they will be handled on the device here.