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A hierarchical decomposition of a project into deliverables and then into work small enough to own and estimate. Phases at the top, tasks at the bottom.
It is a structure of outputs, not activities. "Approved design" is a deliverable; "work on design" is an activity and cannot be finished.
Until each item has one owner, a size somebody can estimate with a straight face, and an unambiguous finished state. For most work that is a few days to a couple of weeks.
Going further creates a plan that costs more to maintain than it informs. Half-day tasks in a six-month plan will be rewritten before they are reached.
Small enough to own and estimate, usually days to a couple of weeks. Smaller is maintenance overhead.
Deliverables. Activities cannot be finished, only stopped.
The epic-to-story hierarchy is the same idea under different names.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.