Security check or Security clearance?

When applying jobs, there is a question "Have you had security check or security clearance? If yes, at what level?"
I believe that there must be some kind of security check or security clearance during the process of Greencard application and citizen application. But I do not know what are the levels.
Does anybody know?
For the people who have greencard, what kind security check he must pass?
For the people who are citizen now, what kind security check he must pass?

Thanks!

所有跟帖： 

• 回复：Security check or Security clearance? -863211- ♂ (731 bytes) () 08/02/2010 postreply 20:35:57

• Appreciate you help! -xiling- ♀ (20 bytes) () 08/03/2010 postreply 13:08:55