People and organization contacts, as well as client groups, have a client identifier field that you can use to store their client codes. You can find this under the contact’s Basics section (you can find this in their Details tab).
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You can even search for your contacts or client groups by their identifier number. Just type it into the Global search or the Contacts overview page search.

Keep in mind that Client Identifiers have to be unique and cannot be assigned to multiple contacts.