machineId on user profile

in the user profile api there is a machined property
Is that the mac address for the machine?
is it unique per machine?
How easy is it to fake it if we want to make sure that the user logged in use only one specific machine

The machine ID given by this command IS a consistent unique ID, and it should not be something a user can fake.
It is possible to use it like you describe, however, I would recommend to also discuss this with your devrel, since there can be some unexpected complications if you base your entire app on this (also, so they can ensure that it doesn’t cause a bad user experience in cases where a user might have lost their machine, etc)