Trailer couplings refer to both the electrical and pneumatic lines that span between, and connect, the tractor unit and trailer.
They comprise:
Primary Electrics (24N - ISO 1185) UK only
Secondary Electrics (24S - ISO 3731) UK only
ABS/EBS (ISO 7638-1) Mandatory across all of Europe, including the UK
15 Pin Electrics (ISO 12098) Europe only
Red emergency airline
Yellow service airline
The airline coupling type varies slightly between the EU and the UK.