Combination car seats are often mistakenly called convertible car seats or booster seats. However, unlike a convertible car seat, combination seats cannot be used rear-facing. Unlike booster-only seats, the combination car seat has a harness that is used for children under a certain weight, but can also be used in booster mode later on.
The most common combination car seats can be used in the harnessed mode until your baby reaches 40 lbs. Then you can switch to booster mode by removing the harness straps and using the seat with your vehicle seat belt as a belt-positioning booster through 80 or 100 lbs. If a car seat box says that the seat can be used from 20-100 lbs, or 20-80 lbs, check the labels on the sides of the car seat to find the maximum harnessed weight. With combination car seats, parents often assume that the harness can be used to the maximum listed weight for the seat, not realizing that the upper weight limits are for booster mode only. Using an internal harness beyond its intended maximum weight could result in car seat breakage in a crash.
Several newer combination car seats have harnesses that are rated beyond the usual 40 lbs. These higher harness weights allow your baby to remain in a 5-point harness longer, which is a big safety benefit.
When shopping for a combination car seat, look for a reasonably tall shell, the highest harness strap slots you can find, higher harness weight limits, and wide shoulder belt slots for booster mode. The tall shell and high harness slots will allow your baby to use the car seat longer, since baby's ears will need to stay below the top of the shell and the harness straps must be above baby's shoulders. The wide shoulder belt guides will be helpful in booster mode, because the slit-type shoulder belt guides found on many older combination car seat models don't allow the shoulder belt to slide freely, possibly allowing dangerous slack in the seat belt.