This article has explains how parametric and non-parametric objects are represented in ZEMAX both in the layout and for ray-trace. In summary:

• Most non-sequential objects in ZEMAX are parametric
• The facets are only used to render the parametric objects. For ray-tracing, the objects are not tesellated but instead modeled exactly by their equation
• Unlike imported CAD objects, parametric objects can be optimized
• In general, parametric objects are more accurate, ray-trace faster and requires less memory than CAD objects.