Your NUC is offering you two methods for connecting speakers, analog, which uses a H/W CODEC to generate analog output and deliver it to the 3.5 jack, and digital, which includes the digital audio data within the (three possible) DisplayPort (DP) output streams of the processor. In this latter case, an IC converts one of these streams to HDMI format for the HDMI output. In addition, the Thunderbolt/USB-C output carries one of these DP streams.
Your "standard" options are:
Your "expansion" options are:
Hope this covers it completely - and doesn't create more questions than it answers!
Thank you for your comprehensive answer, much appreciated. I am currently using 'Standard' option 1 (plugging PC speakers into the front 3.5mm port), and like you, want to route everything from the back of the NUC. I like the 'expansion' options 1 and 2. I've had a look around for USB and Thunderbolt devices to drive the speakers but not sure what you are referring to - could you give a couple of examples of both please? Also as another option could I just buy a USB Type C to female 3.5mm adaptor and connect the Thunderbolt to the speakers that way?
Well, the choices on USB vary from the very cheap (consider: https://www.amazon.com/Plugable-Headphone-Microphone-Aluminum-Compatibility/dp/B00NMXY2MO Amazon.com: Plugable USB Audio Adapter with 3.5mm Speaker/Headphone and Microphone Jacks) to the very expensive (consider: https://www.amazon.com/Tascam-US-2x2-USB-Audio-Interface/dp/B00MIXF2RS Amazon.com: Tascam US-2x2 USB Audio Interface). I find that many of the cheap devices work pretty darned good, but I have 60 year old ears that were somewhat regularly abused with loud progressive rock music.
Most of the devices/docks that you see for TBT/USB-C include an audio solution that is really not much better than the very cheap USB devices such as I have shown above. There is some higher-end stuff that goes from the ridiculous (consider: https://www.amazon.com/Universal-Audio-Thunderbolt-Interface-Processing/dp/B00Y3Q6BE6 Amazon.com: Universal Audio Apollo 8 Thunderbolt Audio Interface with Quad Processing) to the not quite as ridiculous (consider: https://www.amazon.com/Zoom-TAC2R-Thunderbolt-Audio-Interface/dp/B00NHLB0IU Amazon.com: Zoom TAC2R Thunderbolt Audio Interface).
Hope this helps,