Unit/Range of Proj/Real Coordinates

1.) What kind of units the Real Coordinates are measured in? mm? Are they measured from the device camera?
2.) How about Projective Coordinates? from 0 to 1, right? but I found the Z value still returned the Real coordinate.
3.) How far a user can be detected? When a user was standing from 4 meters, the device seemly cannot detect the user. But I am using Realsense R435, which is supposed to detect up to 10 meters

FWIW - the d435 is not a great sensor out beyond about 3m as its error levels seem to go way thru the roof and seems to make it hard to get a viable skeleton. Which yes seems rather dumb :frowning: