Global rulers appear at the top and left sides of
the illustration window. The default ruler origin is located at
the top-left corner of the illustration window.
Artboard rulers appear at the top and left sides
of the active artboard. The default artboard ruler origin is located
at the top-left corner of the artboard.
The difference between artboard rulers and global rulers is that
if you select artboard rulers, the origin point changes based on
the active artboard. In addition, you can have different origin
points for artboard rulers. Now, if you change the artboard ruler
origin, the pattern fills in objects on the artboards are not affected.
The default origin point for the global ruler is at the upper-left
corner of the first artboard and the default origin for the artboard
rulers is at the top left corner of the respective artboard.