"End client" refers to an employer who uses the system. Typically end clients are a payroll company's clients. If you are a payroll company, your employers are known as "end clients" in the platform.
The terms End Client, Employer, and Company, are often used to refer to the same thing.
Note: Seeing the word "Client" elsewhere? This is not the same as an "end client." "Client" is typically the same as a payroll company or a bureau. That's why in the platform you'll often see the word Client with the word (Bureaus) in parenthesis after it. When you add a Client, you're adding a pay roll company that can add as many end clients as they want to the system. If and when you add and end client, you'll be adding one specific employer, who can't add any end clients themselves.