Lines have an X-Y coordinate-system.

•The Origin of a Line is at its start-Point

•The +X-axis of a Line is from its start-Point(1) to its end-Point(2)

•The +Y-axis of a Line is at +90º from Line.

When we add a dimension that is a perpendicular-distance from the Line, the

•dimension will be Positive if in +y-axis half of the Line (blue area in the image)

•dimension will be Negative if in the -y-axis of the Line (green area in the image)

In the top image:

•Dimensions from the Line(1 to 2) to Points a and b will be positive

•Dimensions from the Line to Points c and d will be negative

The X-AXIS and Y-AXIS of a Part each has its own coordinate-system.

The image to the left shows the Coordinate-System of the Y-AXIS

The local +x-axis of the Y-AXIS of a Part is along the Y-AXIS!

Thus, the local +y-axis of the Y-axis of a Part is at +90º from Y-axis

Thus a dimension that is a perpendicular-distance from the Y-AXIS to a Point is in the green area will be negative ...

... and this is in the +X axis direction of the Part!