This is another term used to define a limited company and is most commonly used with reference to a company that contains a sole director/shareholder that supplies services to an end hirer, often through a recruitment agency.
The PSC (or limited company) supplies the services of its worker, but is legally considered a separate entity to the worker, unlike those workers that operate as traditionally self-employed sole traders.
Workers that operate via their own PSC will often receive payment for their work through a mix of salary and dividends if working outside of the IR35 legislation, or just employment remuneration if working inside IR35.
