副 is commonly used as a prefix before other kanji with a meaning of 'sub-' or 'deputy'. For example a subordinate department (副区分), vice-president (副会長) or in IT for the term subquery (副問合せ).

It can also have a meaning of secondary, additional or supplementary, which seems more likely given the context of a customer identification number (presumably on a form or something like that).

However, we would probably need more context to give a definitive answer.

To give a definitive answer, we would need more context, but