EPICS are decomposed as the requirement rises in priority. This typically happens in backlog refinement with the Product Owner and end user working together.
EPICS are large grain capability needs. The Capability Needs Statement that is a required artifact of the 5000.87 Software Pathway can be thought of as comprising EPICS Requirements - A high-level capture of enduring need requirements as they relate to the overall threat environment. They might be capability gaps or mission deficiencies or enhancements to existing operational capabilities. They are typically comprised of features that span multiple releases. If these requirements are near term priorities they will have to be decomposed into smaller requirements called user stories that the team can finish inside a single iteration or sprint.
Features are specific capabilities or functionality that you want to include in your release.
A feature is slightly smaller than an epic, typically can be done inside of one release but not inside one iteration or sprint so features will also need to be need to be decomposed into user stories before you can begin work.
Your product roadmap will typically show your release plan and that is typically shown as features.
User stories are short, simple descriptions of a feature told from the perspective of the user.
These are bite size levels requirements that we can get all the way to our definition of done inside of a single iteration.
So if we have EPICS or Features in the backlog that are rising in priority those will need to be broken down into user stories. One of the main responsibilities of the Product Owner is to make sure that happens in backlog refinement.
And then lastly, if the team wants to, they can break the stories down into tasks. Typically when we break things down in tasks, we try to keep them inside of a single day, 2 to 8 hours. And if we do break them down into tasks a lot of times we assign estimate hours to those tasks and keep up with the remaining hours as work progresses so we can track our sprint burndown.