The requirement to register a C4C company arises when the Hiring manager decided to allow the third party company contractors accessing to Shell application resources via C4C service to perform their works and this third party company is not yet registered in C4C. The request to sign the legal agreement (with C4C clauses) is to send to the contract holder via email.
Refer "C4C Registered Company List" in Important Documents on the registered contracting companies that has signed up for the service (only Shell staff or C4C users that has Sharepoint link access can view the list as this is confidential data). The record is ever-greened and updated as soon as a new company is successfully registered in Shell domain.