The Ray-aiming is an iterative ray tracing algorithm in ZEMAX that finds rays at the object that correctly fill the stop surface, for a given stop size. It is generally only required when the pupil, the image of the stop as seen from the object space, is considerably aberrated or shifted/tilted.

The Ray-aiming option is specified under System > General > Ray Aiming menu.

Without Ray-aiming, real rays from the object are aimed at the paraxial entrance pupil; the paraxial image of the stop seen from the object. The size and the location of the entrance pupil is reported under the general section of the Prescription Data (Report> Prescription Data in the main menu). If there are no optics between the object and the stop surface, or if the aberration introduced by the lenses between those surfaces is negligible, then the real and paraxial marginal ray height at the stop surface will be very close, hence no pupil aberration.  In this case, Ray-aiming is not required and enabling it will only slow down the raytrace without any increase in accuracy. Only use Ray-aiming when necessary!