Where Member ID can be stored

Summary:

Our company insurance is with Vendor A and will be changing to Vendor B. Vendor A uses the HCM person number for the insurance Member ID. Benefits is considering with Vendor B to generate the Member ID and provide that to us. Is there a seeded field somewhere to store that 11 characters? In the Benefits Service Center we found under Person Info a field called Plan. Is that field Plan to be used for that Member ID or what purpose does it have? Where ever we store this Member ID we would want