The XMPP way is to have "disposable", or at least easily substituted, clients, maintaining long-term state on the server, and allowing it to be synchronized between clients. In particular, this can be seen on how the roster and presence fan-out operate - clients defer the operation of such things to the server, which manages the shared state and allows servers to access and manipulate it.
Historically, however, we have not done this for some more recently designed services, including Multi User Chat and PubSub. In both cases, different clients may be unaware of what chatrooms (etc) are joined (etc) by which other clients. This causes practical difficulty in seamlessly switching between devices and/or clients.
When a client wishes to subscribe to a node, either on the local server or remotely, using this protocol it does so by sending an <iq/> of type "set" to its own account, containing a pam element, which in turn has a service attribute (the target service jid) and a payload of a Publish-Subscribe (XEP-0060)  subscribe element (as described in Publish-Subscribe (XEP-0060)  §6.1). Example 32 from Publish-Subscribe (XEP-0060)  is thus performed in this protocol as follows:
Note that because the Publish-Subscribe (XEP-0060)  operation is intact within the pam element, local servers MAY interpret the operation, or MAY forward it verbatim. Note that the client SHALL always use its own bare jid (eg, <email@example.com>) within a subscribe, servers MUST verify this.
When the remote service replies, the local server SHALL first notify all joined clients of the new subscription (described more in #sublist)...
... and then MUST respond to the original <iq/>. Since the subscription has already been notified, this is an empty result <iq/>.
If the local server detects an error, it MUST NOT forward the request, and MUST respond with an <iq/> stanza of type error, which contains an error element which MAY be stamped with the local server as generator. Thus Example 34 from Publish-Subscribe (XEP-0060)  would be very similar:
If the remote service rejects the subscription request, the local server simply forwards the response back as an <iq/> of type error, with the remote error copied through. The generator MUST be set to the remote service if missing. Thus Example 35 from Publish-Subscribe (XEP-0060)  might look as follows:
Clients MAY assume that if the generator is missing, the error is generated by the local server and not a remote service.
Clients obtain a current listing of the subscriptions, for example on initial connection, by sending a subscriptions request qualified by the pam namespace. If a client already has the opaque version identifier cached, it MAY include it within a "ver" attribute:
Alternately, a server MAY - if the client has supplied an opaque version identifier - send a sequence of <notify> elements followed by an empty <iq/> result.
Clients MAY persistently store the last "ver" attribute seen from either the <subscriptions> response or the last <notify>, whichever is later. This can then be used to minimize the volume of subscription data transferred during resync.
Servers need to subscribe to remote PEP services explicitly those nodes which are of interest. Interest needs to be detirmined by the client issuing a request; but this implies that servers would gradually acrue any node type which the user has had a capable client at any time.
Perhaps timing out node types which have not been requested for over a certain period?
Clients can use +notify to handle auto-subscriptions between clients and their server.
Servers receiving +notify from accounts known to support this protocol ignore them.
Clients filter subscriptions using a specific stanza (iq, probably), containing a list of node names. This can be used instead of the odler +notify (which is broadcast).
We probably want to say that events are now archived by MAM, but this may imply that clients need to filter out such events (or explicitly include them). Maybe the mask above affects MAM queries?
I have literally no idea. I don't think anything new is introduced that couldn't be discovered by traffic monitoring, although it collects and collates information that previously would not have been so readily available.
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Added some concrete protocol around subscription tracking.
Initial version approved by the Council.