I saw in the APIs a column named registrationIpAddress which was the IP address recorded when the user registered.

Under modcp, IP Address Tools, when we query a member, does it use the latest IP address or the registration IP Address or both to link all members who are using the same IP address?

It would show all IPs used by that user, if seaching by user. It would show all users who use that IP address, if searching by IP address