Minor The wording of some of the paragraphs in this section is a bit confusing (and has grammar errors)
a) There are sentences which say "When a new Non-Shared Subscription is established, the last retained message, if any, on each matching topic name are sent to the Client. These messages are sent with the RETAIN flag set to 1." Neither of these statements is necessarily true as the behaviour now depends on the value of Retain Handling. Also "are sent" should be "is sent"
b) These sentences are followed by "Which retained messages are sent is controlled by the Retain Handling Subscription Option. Refer to section 3.8.3.1 for a definition of the Subscription Options." This is ok, except that they are immediately followed by a bulleted list describing the Retain Handling Subscription Option. It would flow better if "Refer to section 3.8.3.1 for a definition of the Subscription Options." were moved to after the bulletted list
c) Second item in the bulleted list is "• If Retain Handling is set to 1 and the subscription did not already exist, the Server MUST send all retained message matching the Topic Filter of the subscription to the Client [MQTT-3.3.1-10]." In this sentence "message" should be "messages".
d) Third item in the bulleted list is "• If Retain Handling is set to 2, the Server MUST NOT send retained messages at the time of the subscribe [MQTT-3.3.1-11]." This is ambiguous. I think what it is trying to say is "MUST NOT send retained messages as a result of the subscription" or something like that.
e) Also in 3.3.3.2.4 it says "If a Server receives a CONNECT packet containing a Will Message with the Will Retain 1, and it does not support retained publications..." In most other places we talk about "retained messages" not "retained publications".