A "Client" is typically a payroll company or bureau which can add "end clients" to the system.
This is a high level role, and only Super-Admins can add new Clients. The structure looks like:
"Client/Bureau" --> "End Clients/Employers" --> "Workers/Employees."
Note: Don't confuse "Client" with "End Clients." Remember that "End Client" refers to employers. Typically end clients are a payroll company's clients. If you are a payroll company, you'll be adding all your employers as end clients.