Topic
Stacked stale notifications help nobody. The Topic request header labels a Web Push message so a newer submission with the same label replaces the older one still waiting at the push service, and a reconnecting device receives only the latest version.
Usage
An application server adds Topic to the POST submitting a push message. While a message with the same topic sits undelivered at the push service, a new submission overwrites the stored one instead of queueing behind it. Delivered messages are untouched: the replacement window is exactly the retention period the TTL header buys.
Scores, unread counts, and status updates fit the pattern, where only the latest state matters. Chat messages do not, since each carries distinct content, and submissions without Topic queue normally.
Values
At most 32 characters from the base64url alphabet. The label is opaque to the push service and scoped to the subscription, so two subscribers never share a replacement window.
Example
A sports-score update replacing whatever score message still waits for the subscriber.
POST /push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1
Host: push.example.re
TTL: 7200
Topic: match-4817
Content-Length: 112