I've got questions about subscriptions too, but let's see if this works:
A "public" account --really just an Access Level -- is available on some (not all) nodes. These are nodes that may be routing public traffic to other nodes.
You do not need to be in MSR of some distant node -- there only needs to be a string of these routing public-access-enabled nodes between you and the destination, and you get a subscription at the destination node.
SOME nodes, such as the node at the local Starbucks, actually have something you can usefully DO in their public-access-enabled area, such as the "Make your own Soy Latte" menu, so you can order-to-go without bugging a meat body about it, or being on-site to use their AR system. I think the example in the book is booking your flight at the airport node. THIS requires a subscription, because you're actually communicating with this node (i.e. it is your destination node).
I look at it this way:
You have a persona. Your persona (generally) is only on your commlink.
Then you spawn "icons" (subscriptions) that hook-up on destination nodes on the web. These route through publicly available public-access-nodes -- whether in a streetpost, or at a megacorp's front foyer. It does not subscribe to these nodes, it is simple data traffic at that point, and splits up into bitty packets of data until it reassembles (think Star Trek transporter) at the destination node.
You spawn these "destination node icons" whether in AR or in VR. On your end you are either swapping between these destination nodes in VR OR you are viewing different "windows" in your AR interactions. Your commlink has a maximum number of subscriptions (windows or icons) that you can have "open" at once. That's how many "destination nodes" you can have a connection to at once. So you can be logged in at Dictionary.Node, Starbucks.Node, Sea-TacAirport.node, Ares Macrotechnology Personal Defense Ordering System.Node, etc. all at once, whether you're viewing them "in first person" via VR OR you're fiddling in floating windows that you can move around within your AR field of vision.
That help? Hope so....