Membership ID is associated with private data only viewable by yourselves, SITA and your registrar.
Registrant contact records (as well as billing, admin and tech record) are part of a public Who-is service and any third party using Internet can access this information.
As a matter of policy, registrant and Membership ID holder must be the same legal entity. Registrars are responsible for verifying the fact and may refuse to make changes if these are not consistent.
However, the contact details do not necessarily have to be the same in both records. For example, you may decide to share with SITA in your Membership ID record the name of a person responsible for eligibility verification of your company, but for example to protect the privacy of your employees, you may decide to show in the registrant record a functional email address and a name of a department responsible for maintaining the name. The public information must be correct and functional.