Change:
Error? / A stateless reset is not appropriate for signaling error conditions./ I understand, but this sentence seems odd to me, is it possible to say /error conditions when there is connection state/ or something like that? Is it worth a cross ref to section 11 to be clear what a “normal” error is? Section 9 NiTs in wording: /Acknowledgments cannot be used for path validation as they contain insufficient entropy/ Acknowledgments cannot be used for path validation because they contain insufficient entropy/ /as endpoints independently determine reachability on each direction of a path./the endpoints independently determine reachability on each direction of a path./ Is it slightly easier to read: / One or more frames are assembled into packets./ / One or more frames are assembled into a QUIC packet./ In section 3.3. It says: /The receiver of a stream sends MAXSTREAMDATA (Section 19.10) and STOP_SENDING frames (Section 19.5)./ Is this a para where ACK frames should also be mentioned? It seemed odd to me that ACKs are not mentioned at some pointy around here.
I think that the intent is to be more general than just "this is not for connection errors". How about: I don't think that we want to talk about acknowledgements at the level of the stream section. They don't affect stream states and this is specifically about what stream-related frames can be sent.

Discussion:
to transfer to a new network path. Frames are used in QUIC to communicate between endpoints.  One or <del> more frames are assembled into packets.  QUIC authenticates all </del> <ins> more frames are assembled into a QUIC packet.  QUIC authenticates all </ins> packets and encrypts as much as is practical.  QUIC packets are carried in UDP datagrams (UDP) to better facilitate deployment in existing systems and networks.

Path validation does not validate that a peer can send in the return direction.  Acknowledgments cannot be used for return path validation <del> as they contain insufficient entropy and might be spoofed.  Endpoints independently determine reachability on each direction of a path, and therefore return reachability can only be established by the peer. </del> <ins> because they contain insufficient entropy and might be spoofed. Endpoints independently determine reachability on each direction of a path, and therefore return reachability can only be established by the peer. </ins> Path validation can be used at any time by either endpoint.  For instance, an endpoint might check that a peer is still in possession

endpoint MAY send a stateless reset in response to receiving a packet that it cannot associate with an active connection. <del> A stateless reset is not appropriate for signaling error conditions. An endpoint that wishes to communicate a fatal connection error MUST use a CONNECTION_CLOSE frame if it has sufficient state to do so. </del> <ins> A stateless reset is not appropriate for indicating errors in active connections.  An endpoint that wishes to communicate a fatal connection error MUST use a CONNECTION_CLOSE frame if it is able. </ins> To support this process, a token is sent by endpoints.  The token is carried in the Stateless Reset Token field of a NEW_CONNECTION_ID