A curvilinear abscissa ("distance along"), optionally with vertical and lateral offset. The units are defined by the associated positioning system. See also ISO19148:2012: LR_PositionExpression. Please note that the concepts match: in ISO19148, LR_PositionExpression refers to exactly 1 distance expression (here: instance of Length), one linear element (here: one reference line, via the reference to positioning system), and one linear referencing method (here: referred to, via the linear positioning system).