What is us security clearance?

Answer 1

Anyone working in a classified Government or Military program is required to have an active security clearance for the level of security they're working at (Secret, Top Secret, TSC, etc.). The clearances are updated every several years by updating background investigations. However, once the person leaves that position, their clearance becomes inactive.

In the military, clearances are granted and usually maintained by the member as long as they're in, as clearance levels are generally assigned to specific job types and ranks (officers generally hold TS clearances). For example, as a submariner, I was required to have a minimum Secret clearance just to be a part of the crew, but my specific job required specific instances of temporary TS clearance.

When inactive, a past clearance can be reactivated (depending on the time between jobs) should a person take another job requiring one. This is why Veterans who held clearances while on active duty are typically hired by defense contractors over those who've never held one - the company only has to pay for the BI update (or nothing if it's recently been held), as opposed to a complete BI for someone whose never had one (which can take up to a year, meaning they can't be assigned to classified work).