If you are looking for a new job and writing your resume, I suggest that you write "sent by the sending company to a company to do a certain job". If it is some formal forms, such as the registration form for the exam, you should write the sending unit.

Outsourcing personnel refer to some companies that contract some business that does not involve trade secrets to a professional company in order to save costs, and the professional company sends personnel to work in the agreed company.