Glen,
Not sure if you are referring to the state (Closed vs Resolved) or the actual "Close Code" field. There is no "Resolution Code" out of the box so if you have added one, that would be up to your organization of why they think they need an additional field and what they are trying to track. The Closed Code is meant for metrics such as feeding into Problem Management (i.e. Solved with workaround)
As far as the states, this follows ITIL best practices. A "Closed" ticket is one that is closed meaning per ITIL it cannot be re-opened. Therefor you have "Resolved" which ultimately pauses your SLA clock waiting for confirmation from the end user that the case has indeed been resolved and thus can be closed (or timed out for auto-closure). There is also an out of the box business rule that will handle the auto-closure of Resolved tickets based on elapsed time.
