In Jira Agile when should I use a "Story" and when should I use an "Improvement"

We just started using Jira (with the Jira Agile plugin - Scrum template) for our issue tracker and agile planning.

I am confused about the difference (or intended difference) between story and improvement. All our "stuff" is written as stories. Those stories describe both new functionality and improvements to existing functionality. Should those all be listed as a Story or should the improvements be listed as an Improvement?

I think we should keep things simple and just have Epic, Story, Bug; however, I don't want to hide Improvement and end up with some unintended effects or end up missing useful functionality.

Thanks in advance for your advice.

We use "improvement" for functionality improvements in user stories delivered in previous sprints. In other words we use "story" for new user centric functionality and "improvement" for functionality improvements of current features and non-user centric functionality.

hope it helps