They replicate a dot matrix printer which prints the message.

I’ve been using an XU4 for a number of years. Not used it as a server but it works great as a client. I’m sure it would with excellent as a server. I’ve had Ubuntu, tried Android, and currently running Batocera for gaming.

I like that it has an SD slot like a Pi but also a storage module which plugs onto the board which is much faster. I can boot from one or the other by flicking a switch on the board.

Only draw back is that it doesn’t have onboard WiFi or Bluetooth and limited USB ports. I had to use a powered USB hub then find a PSU with a step down inverter to power it all, making it bigger than a small board. I’d still highly recommend it though.