The absolute coordinate system here is essentially the same as the absolute position in physics. Here, the coordinates of points based on a coordinate system are absolute coordinates relative to this coordinate system.
The absolute value is the value relative to the origin of 0 in the number rule.
The reference here is artificially specified, but it can also be used as a reference. All numbers can get their absolute values from the origin 0.
For example, the absolute values of -3 and 3 are both 3.