Topic: what it does in a system design

Topic

Takes each published message once and puts a copy on every subscriber's own backlog, so a slow subscriber only delays itself.

Instance tier: Sizing label for this topic instance (small to xlarge). It is shown in the metrics; in the current simulation it does not change delivery speed.
Max waiting messages per subscriber: How many copies may wait for each subscriber. When one subscriber's line is full, new copies for that subscriber are dropped silently and the publisher is never told. A subscription can override this.
Attempts per message: How many times a message copy may be handed to a subscriber's workers before it is dead-lettered (set aside and lost for that subscriber). A subscription can override this.
First retry after (ms): How long to wait before retrying a copy whose worker failed. Each later retry waits twice as long as the one before. A subscription can override this.
Visibility timeout (ms): How long a subscriber's worker has to answer before the copy is handed out again, possibly causing duplicate work. Too short means duplicates; too long delays recovery. Default 30000.
Drop messages older than (ms): Copies still waiting after this long are dropped unseen, so a slow subscriber skips stale messages instead of working through them. Empty means copies wait forever.
Subscriptions: Per-subscriber overrides. Each subscriber gets its own copy of every message with its own line and retries, so a slow subscriber only grows its own backlog. Unlisted subscribers use the topic defaults.