The Planner can be used in multiple scopes.

In Project scope, you work with all releases and sprints defined in the Release tracker. This is the biggest scope, which may be useful when re-organizing the release hierarchy, or moving work from the project backlog to release and sprints. You can open this scope by clicking the Plan Releases link in the release list on the right panel.

In Release scope , you plan a single release together with its sprints .You can setup a highly granular release hierarchy and cadence.

In a Sprint (sub-release) scope , you plan a single sprint or sub-release.