Lines have an X-Y coordinate-system.

•The 0,0 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 you add a dimension that is a perpendicular-distance from the Line, the:

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

•dimension is 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 are positive

•Dimensions from the Line to Points c and d are negative

The X & Y-axes of a Part also have their own coordinate-system.

The bottom 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!

In the bottom image:

A dimension from the from Y-axis to Point (a or b) in the Blue area is Positive.

A dimension from the Y-axis to a Point (c or d) in the Green area is negative.