Buckle – é só a fivela do cinto

Acredito que o termo se refere a todos os tipos de fecho do sistema de retenção.

Achei estes exemplos através do google – talvez ajude

1. In case of an accident, passenger safety is increased by *retention systems* ….. The
retention systems include, in addition to driver and front passenger airbags, side and head (curtain) airbags, and safety seat belts with pretensioners and limiters, also a child seat…
http://new.skoda-auto.com/Documents/EnvironmentTechDev/ EnvironmentTechDevSafety.pdf.

2. Aparentemente, todos estes itens ( e muitos outros) podem ter ‘fechos de retenção’:
Hood *latch* system
Door *locks* and door *retention components*.
Seat belt assemblies.
Seat belt assembly anchorages.
Windshield mounting.
Child restraint systems
Child restraint anchorage systems.

Seat Belt Mistake #1: False Latching
With some seat belt buckles, consumers will hear a “click” when the belt is buckled – but it is not securely fastened. Unfortunately, the first time they find out ...
In some case, false latching can even lead to ejection from the vehicle …false latching defects or failures are caused by poor design and inadequate testing of the seat belt *buckle* or *latch plate*, either at the car company, supplier, or both.
http://www.carsafetylawyer.com/our-practice/seat-belts

hope this helps :)