When using first-order optical parameters, take great care of definitions. Some, like ISNA, are linked to a purely paraxial definition, and can be misleading if the optical system is not well described by paraxial optics. The underlying assumptions of paraxial optics are that the ray makes a small angle and small height with respect to the chief ray, so that

• Snell's Law can be replaced with its linear approximation
• The surface shape can be ignored, and a flat surface of equivalent power is used instead
• The ray slope is equivalent to the ray angle

These approximations are all made to make the numerical computation easier, but at the cost of generality. Parabasal rays are real rays that satisfy the paraxial condition, i.e. that they make a small angle and have small height with respect to the chief ray, but are otherwise traced as normal.

In summary, paraxial ray data is computed using first order approximations to the surface power for tracing rays, while parabasal rays are real, exact ray traces close to a chief or reference ray. Most paraxial data, such as EFL, F/#, and magnification, use paraxial rays and the data is invalid if the optical system is not well described by the vertex power of every surface.

Most analysis features in ZEMAX use parabasal rays, to allow these features to work with a greater range of optical systems, including those with optical surfaces not well described solely by their vertex surface power.

References
1. ZEMAX Users Guide, Chapter 3, "Conventions and Definitions"

Further Reading
Introduction to Lens Design, With Practical ZEMAX Examples, GEARY, Joseph M, Willmann-Bell Inc
Practical Computer-Aided Lens Design, SMITH, Gregory Hallock, Willman-Bell Inc