The test insurers apply
Who owns the vehicle, how it is used, and how often. A vehicle owned by the business belongs on a commercial policy. A personally owned vehicle used mostly for personal driving with occasional business errands usually stays personal. Regular business use — deliveries, service calls, hauling, client transport — is commercial use, and heavier vehicles and vehicles with commercial equipment are commercial regardless.
Why it costs more
Commercial policies rate on business use, carry higher liability limits that contracts require, and list drivers and vehicles. The price reflects the exposure the personal policy was excluding.
The employee case
If staff run errands in their own cars, your business can be sued for an accident even though it does not own the car. Hired and non-owned auto liability covers that, either on a commercial auto policy or endorsed onto general liability or a BOP. The employee's own policy still covers their car.