The acronym for the European child-seat anchorage standard is ISOFIX.
ISOFIX is based on ISO 13216, an international standard that defines fixed attachment points for child restraint systems in passenger cars. Instead of relying only on the vehicle’s seat belt, a compatible child seat connects directly to two rigid lower anchors built into the seat gap. Many installations also use a top tether or support leg, depending on the seat category.
The standard was created to make child seats quicker to install and less vulnerable to common seat-belt routing mistakes. ISOFIX does not mean that every child seat fits every vehicle automatically: seats still have weight, size and vehicle-compatibility limits. The system is designed for a combined child-and-seat mass of up to 33 kilograms in the commonly cited specification.
A frequent mix-up is LATCH, the U.S. name for a broadly comparable lower-anchor-and-tether system. CRS means “child restraint system,” while Top Tether identifies one anchorage component rather than the name of the whole standard. ISOFIX lower-anchor requirements were published in 1999; top-tether requirements followed in 2004.