Then this just adds more confusion to some of the installations I've seen.
No it's pretty simple. Basically off the VDSL filter the engineer fits there are two IDC terminals on the back which can be used for wiring in a data extension. The cable running from this can either terminate in RJ11 so it plugs straight into the VDSL modem or an RJ11 socket.
Either way, it's still the same thing as being plugged straight into the master socket as it can't be ran off any existing extensions.