The optical system in the region close to the optical axis, we called it as "paraxial region". The treatment of light in this paraxial region is to as "paraxial optics".
The paraxial approximation enables us to computer some of the most basic properties of an optical system with a minimum of calculation in CAXCAD.
Paraxial surface :
Paxaxial ray trace including the first order data is the base of the CAXCAD
Paraxial Ray Trace Data