Meaning of timeout

[gtb104]

For the timeout property, the docs state

timeout (number)
Specify a minimum timeout in milliseconds to use for the transport connections. By default this is dynamically calculated based on the measured RTT and the number of expected round trips. This setting will establish a minimum, but if the calculated timeout is higher, that will be used.

Could I get more clarification on this? Is this how long it waits to receive a RECEIPT response to a SUBSCRIBE message? Is this how long it waits to receive a reply to a SEND? Is that how long it waits for the full MESSAGE to be returned? Something else?