Name what the story owns and what belongs to another owner. Account for the relevant actor, beneficiary, mandate, exact work, outcomes and continuations inside that scope. Reuse another owner's exposed contract when needed.
Why: omissions lose obligations, while invented responsibility creates authority the story never had. Completeness is meaningful only against a named boundary.
Consequence: removing password collection from registration does not decide all later login policy. An entry story can depend on session facts without owning the whole account lifecycle. An automated counterpart does not become the initiating person's contribution, and two department contexts do not create two independent people.
MECE means mutually exclusive, collectively exhaustive: branches do not overlap and together cover the chosen question. It is useful for a bounded analysis, not a claim that one model exhausts the enterprise.