A key can do more than one job.
The blade may open a mechanical lock. A chip can allow the engine to start. Remote buttons may operate doors or the trunk. When comparing a replacement, be clear about which functions are included.
Check before you buy a fob.
The case can look right while the electronics inside are wrong for the vehicle. Before ordering your own part, check compatibility and whether it can be used for the work. If you already bought it, keep the packaging and listing.
No spare does not answer the whole question.
Replacement without an original is possible for many vehicles, but that isn't a promise for every model. Some jobs need different equipment, parts or another service. Confirm the options for your car before deciding on a dealership visit or tow.
Test the finished key.
Try the functions included in the job: starting the car, remote buttons and any separate door blade. Keep a working spare away from your everyday key. Two keys on the same ring can still disappear together.
Year, make, model and the number of working keys.
Cutting, programming and the functions included.
Check whether the job can be handled where the car is.