Some Group 2/3 car seats convert to a Group 3 seat, which is a backless booster seat, also known as a booster seat. You can also buy Group 3 booster seats separately.
Booster seats raise the child's body to a height suitable for use with the adult seatbelt. A booster cushion satisfies the legal requirement for children up to 1.35 metres (approximately 4ft 6in) but are often not recommended for younger children. That's because booster seats do not have a full-length back and 'wings', which provide extra protection for the head and chest in a side-impact crash.
From 1 March 2017 the rules for using booster seats have changed.
Any newly approved backless booster seat on sale will be limited for use only with children taller than 125cm and weighing more than 22kg. However, other backless booster seats on sale approved for use for children of between 15kg and 36kg. Both seats are legal to use as long as you are following what it says on the label.
If you already own a backless booster seat you can still use it, as long as you're following the age/height on the label.