The System Owner is the individual who downloads the Sovereign Stack code onto their machine and deploys websites to hardware they control. This hardware can exist in multiple geographically distant data centers.
Trusted Mobile User
A Trusted Mobile User is a smartphone or laptop that has been issued a Wireguard profile for connecting back to one or more data center locations. Trusted Mobile Users typically access restricted Sovereign Stack services (e.g., as administrative interfaces or semi-trusted management interfaces like the core lightning native websocket interface. These restricts are via IP whitelists or other authentication techniques.
Local Website Users
Deploying a bitcoin-native intranet website is definitely a valid use case. For example, let's say you are a restaurant owner and you want to make your Bitcoin-native website accessible to clients connecting from your WiFi network. This could include servers or waiting staff to take customer orders. Or in the case of the example Satoshi Spirits website, customers can submit orders as well.
Public Website User
In many cases you will want to expose your Deployment websites to the Internet. Unknown people out on the Internet who access your public websites are known as "Public Website Users" and access the website endpoints via the data plane.
The objective with these users is to provide snappy website access and make it EASY and CHEAP for one to pay you in any amount. Lightning is NECESSARY to achieve this; no one wants to wait 10-60 minutes for a sub-cent transaction to go through, nor do they want to pay more in fees than the transaction is worth.