When using the Essential Balance i-Size car seat in the under 21kg/105cm configuration (with the harness) the ISOFIX and top tether must be used. This configuration can be used until the child reaches 21kg or 105cm, whatever comes first. Once the child exceeds 21kg/105cm the seat can be used with the harness stored within the seat and the child secured in using the 3-point vehicle belt. This configuration can be fitted with or without the ISOFIX fittings. We recommend where possible to also secure the top tether. However, unlike the under 21kg/105cm configuration, this is not essential.