I have a STEP 3D file and when I open through notepad, Its lot of entities and I gone through http://www.steptools.com/support/stdev_docs/express/step_irs/html/schema.html#geometry_schema but I could not understand the entities. Is there a documentation that explains these entities? If so it would be really helpful to understand.
The recommended practice for STEP AP 203 is a good place to start.
The CAX-IF recommended practices may also be useful, along with the schemas and file library.
If you need more information, the STEP Module and Resource Library can be purchased from ISO.