MacPorts (a package manager similar to homebrew or Fink) creates an additional user with the name MacPorts/macports and a group with the name macports. Usually the UID(MacPorts) is the last regular user's UID+1 and the GID=UID(MacPorts).
After a normal OS X install this is 502/502 (if only one user is configured = admin) or 503/503 (if two users are present = admin and standard user).
Though the user has a non-system-user UID (>500), the account isn't visible in the Users & Groups System Preferences because it is marked as disabled. Time Machine/Migration Assistant still recognizes it.
If you don't use MacPorts anymore, you can safely ignore the account and uncheck the checkmark.