Mathematically:
Orthogonal is a term meaning that a line and a plane are perpendicular. The special name orthogonal is given so people immediately known in three space.
FIRST-ly:
The rule basically means that if a box is created of dimensions 28x38x60, your robot should be able to fit inside of this box. If your robot has an origin point, then there are x, y, and z directions from that point. Those vectors create planes which the third vector in the set is orthogonal to. Basically, fit your robot in a box that size. Hope that was helpful!
