Group 2/3 car seats are high-backed booster seats. A group 2/3 car seat will last your child from 15-36kg which is from about 3 years to 12 years. Most group 2/3 child car seats don't have a harness, instead you'll secure your child and the seat using the car's adult seatbelt.
Some models can still be installed using Isofix to keep them securely attached to the car. Some parents worry about moving their child from a Group 1 seat, which uses a harness, to a Group 2/3 child car seat using a seat belt, however there are a few seats available which have extended harness use, or other methods to help a child transition from a harness to a belted seat.