Buffer queue in data integration

[tigercl]

Under what circumstances do buffer queues in data integration come into play? Cache messages only when integration is not available? Or is it normally used?

Also what happens when the buffer queue is full? Drop the latest messages? Or does it drop the oldest messages to ensure that new messages are queued?

These don't seem to be mentioned in the documentation now: https://www.emqx.io/docs/en/latest/data-integration/data-bridges.html