Which vehicle, and which seat.
Seatbelt hardware is designed around a specific vehicle and a specific seating position, so this is the field that narrows a requirement fastest. Give the make, model, production year and variant or trim where you know them — and then give the seating position precisely.
“Precisely” means the row, the side and whether the position is outboard or centre. It is worth stating the side physically — left or right as you face forward in the vehicle — rather than as “driver’s side”, which means opposite things in left- and right-hand-drive markets and is a genuinely common source of error.
If the vehicle is not confirmed, say what is known: the body type, the approximate age, the market it came from. Partial vehicle information is still useful. Absent vehicle information usually means the enquiry cannot progress at all.
Not enough
“Need a seatbelt for a Swift.”
Workable
“Maruti Suzuki Swift, 2018, VXi. Front seatbelt, right-hand side as you face forward. Right-hand-drive vehicle, India.”