The checkbox is used to select items and then perform actions on them. When you check the box, buttons for the available actions become visible.
For shell users the available actions are:
- Delete the shell user
- Change the shell user password
The checkbox is not used to filter the display of applications. The only thing that filters the display of applications is the filter/search field at the top of the page.
If you want to see all of applications that are assigned to a single shell user, then enter the name of the shell user into the search field. When you do, the list of shell users in the top half of the page will be filtered and you'll see that shell user with their apps listed right next to them:
If you want to see the shell user that owns a single application, then enter the name of the application into the search field. When you do, the list of applications in the lower half of the page will be filtered and you'll see that application with its shell user listed next to it:
Regarding "no shell user found", you've asked about that before. My previous answers are here and here.