SIf it's a human being acting => normal user
If it's software acting on its own => service user
If it's software acting on its own => service user
SI just skimmed through the example. As far as I understand it, it's just a demonstration of permissions. The fact that service users are used here does not mean that service users are the general approach you should take. It's just for demonstration purposes.
SJust open another thread here in questions-help-bugs and tell us about it. 
SWell, I use Zitadel for quite a while now. But I would assume there are for sure people that have more experience.
SFor an example/demo, it doesn't matter which user you use. But for a productive application, you should make sure you use the right type of user.