Quick answer
Most boosters are positioned with the vehicle belt, but correct use still depends on the manual, belt path, and child fit.
First, read the manual
A booster seat is not installed the same way as every harnessed car seat. Many boosters are positioned on the vehicle seat and used with the lap-and-shoulder belt. Some have lower anchors to keep the booster in place when not occupied or to improve stability. The booster manual and vehicle manual decide what is allowed.
Use the right belt
Most belt-positioning boosters require a lap-and-shoulder belt. The lap belt should lie low across the upper thighs or hips. The shoulder belt should cross the shoulder and chest. Do not route the belt under the arm or behind the back.
Check the seating position
The back seat is the safest place for children. Also check head support. A backless booster generally needs vehicle head support behind the child's head. If the vehicle seat back or head restraint is too low, a high-back booster may be more appropriate.
Common booster use errors
Common errors include using a lap-only belt, letting the shoulder belt sit on the neck, placing the lap belt on the stomach, using a backless booster without head support, and keeping an unoccupied booster loose when the manual says to secure it. A local certified child passenger safety technician can inspect the setup if you are unsure.
Before you decide
Do not make this decision from one number on a search result. Check the child restraint label, the car seat manual, the vehicle manual, and your state law. Then test the setup with the child actually sitting in the vehicle. A booster or seat belt that looks fine in theory can fit poorly in a specific back seat because belt anchors, seat depth, buckle stalks, and head restraints vary by vehicle.
If this is a legal question, start with your state page. If this is a safety question, start with the core guide linked below. The safest answer is usually the one that satisfies the law, matches the product instructions, and keeps the belt or harness positioned correctly for the full ride.
When to get hands-on help
Get help from a certified Child Passenger Safety Technician if the belt crosses the neck or face, the lap belt sits on the stomach, the booster slides around, the child cannot stay positioned, the vehicle only has a lap belt in the seating position you want to use, or you are unsure whether a forward-facing seat has been outgrown. A short in-person check can catch problems that are hard to see from a product photo or a general article.
Helpful next steps
Sources used for this topic
This article follows the same source hierarchy as the core guides: NHTSA, CDC, AAP/HealthyChildren, IIHS, federal standards, and state law pages. Check the linked core guides for the source list and state-specific notes.