The Gravity app isn't incredibly accurate, but it's better than no scale at all. One of the first problems he and his team faced was that Apple's 3D Touch API measures weight on a custom scale from 0.00 to maximum possible force, with 1.00 as the average touch. They solved this by asking users to calibrate the app using objects of a known weight they would have to hand (US nickels).
The SID (Security IDentifier) is a unique ID number that a computer or domain controller uses to identify you. It is a string of alphanumeric characters assigned to each user on a Windows computer, or to each user, group, and computer on a domain-controlled network such as Indiana University's Active Directory.
The domain identifier portion of a SID issued in Noam is unique to Noam, so the SID for Alice's account in Euro has a different domain identifier. The relative identifier portion of a SID is unique relative to the domain, so if the domain changes, the relative identifier also changes. Thus when a User object moves from one domain to another, a new SID must be generated for the user account and stored in the Object-SID property. Before the new value is written to the property, the previous value is copied to another property of a User object, SID-History (sIDHistory).