I am currently in H1B and my contract says that I need to be a legal alien. Can you please tell me who is a legal alien? Does H1B comes under legal alien or only Green Card comes under legal alien.

Federal Govt. specifies that they need to have a security clearance for citizen's as well as legal aliens. What do they examine in Security Clearance? (Like Social Secuirty, criminal background, credit check, Employment Reference etc)

Thanks in advance for your help!