Rendering

I am new to the software and learning. I have used Solidworks 360 and Keyshot 7 in the past for rendering. Funny thing I noticed with Simlab is when I want to apply a material to a specific surface for example adding lights to a spaceship bulb lens only it applies the material to an entire chunk of an assembly or part instead of the certain specific surface. Is there something I need to select somewhere to make that happen? I like Simlab because it is less processor hungry and doesn't seem to crash like Keyshot does especially with a very complex and detailed model. I have Composer Version 8 if that helps. I tried the internet but couldn't seem to find anything on that topic. Thank you for any advice you can give me.

Regards,

David