Nothing different than if there was a subscriber on that channel. Publish (senders) and subscribe (listeners/receivers) are completely decoupled so there is no dependency between the two.

All published messages are cached for quick retrieval when a subscriber returns from a network disconnect. In other words, if the client is configured for message catch up, those missed messages can be recovered when the client comes back online an resubscribes to the channel. 

Messages are cached for up to 20 minutes and limited to the last 100 messages. If Storage & Playback feature is enabled for the key set (pub/sub keys), then all messages are stored for as long as the configured retention time. This allows a client application to get messages (using the history API) that were sent while that client was offline for a long period of time.

See also: