It's nothing Governmental, but it uses the same cars as the Government. (Discluding the Pacifica, which wasn't there until 2017.)
Nope, another hint. It often is the leader of a line that leads to specific places, which the ones behind are civilian cars.
I guess because of the other "same" threads. Is it again based off a classic car, that doesn't include Bel Air?
Yes, it is based on a classic Chevy, and also built on a newer version of that Chevy. (Probably a dead giveaway)
Nope. El Caminos weren't still made by the time this came out in 2003. What Chevy had a major anniversary in 2003?