What's the difference between a convertible and an all-in-one car seat?

A convertible typically covers rear-facing and forward-facing harness modes. An all-in-one (sometimes called 3-in-1 or 4-in-1) adds high-back booster and often backless booster modes, theoretically lasting from birth through roughly age 10. Specialist communities like r/CPST caution that the 10-year claim is optimistic — by the booster stage, the seat is often heavily worn, and standalone boosters are inexpensive.