OFFSET is the perpendicular distance in millimetres from the measured points to the dimension line. Its sign selects which side the dimension line falls on: positive offsets lie to the left of the direction of travel from P1 to P2.

DIRECTION selects what is being measured and defaults to 'aligned':

• 'aligned' — the distance between the two points.
• 'horizontal' — the difference in $x$ alone.
• 'vertical' — the difference in $y$ alone.

A horizontal dimension between two points at the same $x$, or a vertical one between two points at the same $y$, measures nothing and is an error.

LABEL overrides the measured value with a literal string, for the cases every drawing eventually has — a nominal size, a range, a note in place of a number. Without it the backend formats the measurement it computes. A dimension therefore carries no text of its own by default, which is precisely what keeps it truthful when the geometry changes.

Source Code: draw.Drawing