Previously, it was only the common SLA configured for any agent response (both first and subsequent). However, the first reaction should often be quicker than the rest of the agents' responses so that your customers understand that their cases are received and already being handled. Once they have the first response, customers are prepared to wait for a comprehensive solution to their issue a little longer.
You can now set separate reaction times for the first and subsequent responses, as well as separate settings for non-compliance.