STEP, the Standard for the Exchange of Product Model Data, is a comprehensive ISO standard (ISO 10303) that describes how to represent and exchange digital product information. Reference 4 contains a lot of information on STEP.

For the ZEMAX user there is little to choose between IGES and STEP. Both work well with ZEMAX, and your choice between these two standards is likely to be based on the quality of your CAD program's export routines. IGES is the older format, and several CAD vendors use their own export translator, which leads to some variation between vendors. STEP is newer, and most CAD vendors use bought-in libraries like Step Tools, which means that there may be more uniformity between different CAD packages' implementation of STEP.

STEP objects are accessed and behave exactly like IGES objects in ZEMAX.