Local state laws preside and are all a bit different. Manufacturer recommendations apply for the particular car seat in question as well.
I know my 5 and 7 yr old kids still ride in booster seats in the expedition, but when they go in the back of my ranger, they only wear the seat belts. Boosters seats are not meant to be used with just a lap belt. But when my 3 yr old rides in the ranger, his car seat gets buckled in the front seat, with the airbag turned off. When the child is an infant, the car seat must be rear facing as well.
I was lucky, my wife is a car seat Nazi and figured it all out for us. It's expensive to go through all the different seats through the ages with 3 kids, especially now that these things have expiration dates (the car seats). I probably spent over 1500 bucks on seats alone so far, which doesn't seem like that much now, but when we were struggling to buy formula and diapers, it was tough.