Minor Lines 2135, 2194, 2257, 2317, 2570, 2689, 2818, and 2896:
Change "This property can be used to provide additional diagnostic information." to "This property can be used to provide additional information."
Diagnostic is a little bit misleading, because the user properties can be used for other purposes as well.
Example 1: user property is used to specify a "retry after" interval, so that the Sender can wait for that period and then continue sending messages, instead of forcing it to disconnect and perform the resource intense reconnect.
Example 2: user property is used to set a message source/type
Example 3: user property is used to set priority
This issue was originally reported in MQTT-417, which doesn't seem to be applied in WD14.