Burn on Read

Burn-on-read (BOR) sets the amount of time after a recipient views the content before it is destroyed; it starts counting as soon as content is marked as "read" but will never extend the life of the content beyond the destruct time determined by the "Expiration" value.

Expiration is the maximum allowed time a message will live in a 1:1 or group conversation, and Burn-on-Read (BOR) is how long a message will live once the recipient(s) have seen it.