Appliance Server

This could be anything from smart home devices to HTPCs.  Pretty much, any app or small suite of lightweight apps will fall into this category.

In most cases, you'll want a cheap SBC to cover you here.  Since Raspberry Pis are actually more meant for learning, we're most likely talking about OrangePis, ZimaBlades or something like an Arduino board.  In rare cases, you might have to go for a full blown mini PC.  It really depends on the purpose of the PC.

I don't have any clear winners here or even some "works in most cases" winners because, as with mini PCs, there's just too many competitors and things just change way too often.  Ultimately, you'll need to decide what features are vital to the task and shop around based on that.  Again, you can always ask Reddit, if you're struggling.