To put it simply, when a company employs a clerk, the employment agreement stating the terms and conditions like working hours, salary, medical benefits etc is called a "contract of service". "Contract of service" is defined under the Employment Act as
"contract of service" means any agreement, whether oral or in writing and whether express or implied, whereby one person agrees to employ another as an employee and that the other agrees to serve his employer as his employee and includes an apprenticeship contract;
On the other hand, if the company wants to engaged a security company to provide security services, the agreement between the company and the security company is called a "contract for service", i.e. the services of a contractor.
Use the Internet to do some research on this if you are really interested to know more.
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