Thanks for the compliment, I think visual aids and consistent color coding help to reinforce the concept behind this thread. I'm now thinking that keeping the digital receiver as a separate item is pointless in the visuals (as per my note below).
I'm not sure if syntax items are the only things left to be discussed. For example, I'm not entirely sure that this statement is accurate:
"Source/Transport : USB Host Controller on computer motherboard
-> USB Audio 1.1 Stream over USB cable
Digital Receiver : PCM2704 USB receiver inside Stello DA100"
Is a PCM270x really a digital receiver or is it the source? The definition of source used here is the reading of the content and outputting of a digital stream. This is vague enough that it's hard to define the boundary of the system for computers (since it's all digital internally). I would argue that if an internal soundcard with a digital output is considered a "source" then a PCM270x should be considered a source. I think the boundary for a computer source should be where devices are no longer under the control/management of the computer (hence why I diagrammed it that way).
If my argument is valid, then it makes sense to fold the digital receiver into the DAC device for visual purposes. This eliminates it from a simple diagram and might result in something like this in a full diagram:
I can see how an argument can be made to consider a PCM270x a digital receiver for DACs with built-in USB support if you want it to be a requirement of the chain. In this case, it seems like a chain with say a Trends UD-10 to a Channel Islands VDA-2 would have two digital receivers (one in each device). In that case, I would still argue that the Trends is the "source/transport" rather than the USB host controller.
I'm also looking into where the content decoding occurs for a SqueezeBox chain. I think it can occur on either the server or the SqueezeBox, depending on the original content format (i.e. sends FLAC straight to SB to be decoded there, but transcodes Apple Lossless on the server prior to sending it to the SB).