Well that depends on what you want to achieve and how you want to achieve it.
If you want to directly assign specific items or item categories to an employee within your application, you would have to create a link table such as employee_item where you can connect those two to each other. In this situation you can specify that employee A can see items of category A/B/C, but employee B can see items of category C/D/E. After that you can set up your prefilter to respond in such a way that only the items that exist in employee_item, based on the user that is currently logged in, to be shown.
If you want to limit the items per role, for example in such a way that senior employees can see more items than junior employees, you could set up two prefilters that limit the list of items, and assign those prefilters to the appropriate roles. These roles would be assigned to user groups, the user groups to the user, thus limiting the data that one can see.
Hopefully this clarifies things a bit more 😄