Undelivered Features Fill the Release Backlog
In the "Big Picture" graphic and series and in the product owner blog posts on my Scaling Software Agility blog, we've noted that the primary currency of requirements expressions for the Agile team is the user story, contained in the iteration (story) backlog (see Figure 1).
Figure 1 The Agile team and its iteration (story) backlog.
In a like manner, the release backlog contains the prioritized set of features that have not yet been implemented (see Figure 2).
Figure 2 Product owners and the release (feature) backlog.
Like stories, features can be scheduled (in a release) or unscheduled (waiting for future attention). They are prioritized and estimated. Estimates at this scale are coarse-grained and imprecise, which prevents any temptation to over-invest in feature elaboration and estimating. If and when a feature reaches a priority such that it hits a release-planning boundary, it will be broken into user stories prior to implementation.