A combination car seat is a hybrid car seat. It can also be called convertible but not in the same sense as a convertible car seat proper. A combination car seat is a booster seat that can be used as a forward-facing toddler seat. Parents Central defines it as a forward-facing with a removable internal harness system. Removal of the harness converts the seat into a belt-positioning booster. It can also be defined as a booster with a harness.
If you would like to obtain one, we recommend to look up either booster seats or toddler seats. Additionally, you can purchase a 3-in-1 seat instead. Such a seat functions as a rear-facing car seat, forward facing car seat or a booster, depending on what you expect of it.