That's what it looks like... So it can be anything? It sounds as though RE should be something particular based on what someone is doing/wanting. Or will just simply swapping in the relay in place of the resistor accomplish the task?
You've seen the emitter-follower configuration before, haven't you? We use NPN source drivers in the emitter-follower configuration for column drivers on Charlieplexed common anode displays because it follows the active high column driver signal requirement. Likewise, we use PNP emitter-follower drivers with active low inputs as sinking drivers for command cathode Charlieplexed displays.
However, let's think for a moment about the facts here, disregarding calculations. As near as I can tell from some research, the BJT has been around since 1948, giving people some 62 years to experiment with them. I guess I didn't suppose nobody had ever thought of trying to place an NPN on the high side, given how convenient that arrangement would be for a great many circuits. This leads someone like myself to wonder "why?" Then I see someone post saying that it's possible to do that. I thought maybe I'd just overlooked something since I hadn't seen that configuration yet.
The common-collector or emitter-follower circuit actually has roots going back long before the transistor in the form of the "cathode-follower" vacuum tube circuit configuration...
We use NPN source drivers in the emitter-follower configuration for column drivers on Charlieplexed common anode displays because it follows the active high column driver signal requirement.
Mike, what's the advantage of doing this vs. a "conventional" PNP high-side switch?
The Charlieplexing signal rules (for CA display) require that only a single pin is driven high to select a column and the remaining pins are either hi-z (segment off) or driven low (cathode segment on) so you need a source driver with an active high input (the NPN emitter-follower). Likewise, for Charlieplexed CC displays, you need a sinking driver with an active low input (PNP emitter-follower) to turn on a column and the remaining pins are either hi-z (segment off) or driven high (anode segment on).
If I was driving a traditional multiplexed display I would use PNP or PFET high side drivers or NPN or NFET low side drivers...