A child car seat will either be fitted with the vehicle seat belt or Isofix (or both and top tether).
The car seat featured above is the Britax Duo Plus which allows both seat belts or Isofix installation. Prior to Isofix points being fitted to vehicles as standard, the vehicle seat belt was the only fitting option. Neither option is less safe than the other. Isofix is seen as an option to help eliminate incorrect fitting.