It is important for the child car seat to be an approved one. This means that the seat complies with safety standards and has successfully passed certain safety tests.
Child car seats in the European Union are currently approved under two standards. There is the R44-04 standard, which is based on the child's weight; and the R-129 (i-Size) standard, which is based on height. The i-Size or R-129 approval standard is slowly replacing R44 and will be eventually be the definitive piece of legislation in this regard.
These two approval standards differ in certain aspects. Take a look at the differences between the two here.