For existing clients. Describe what is happening — the ticket gets a reference number immediately, and urgent ones are seen first. If the service is down completely, pick "critical": those tickets raise an alert straight away.
The level sets the order of handling. Exact response times are agreed in the maintenance contract — the following are indicative.
The service is down or unreachable for its users. We start immediately, including outside working hours.
The product runs, but something is broken or there is a workaround. Usually within the working day.
A question, a content change or a requested improvement. Scheduled according to the scope of the contract.