Need-focused. A purpose must express a need of the user, and not just recapitulate a behavior whose significance is unclear. Take the bookmark concept that browsers offer. It’s not helpful to say that the purpose of a bookmark is to “mark a page” or to “save a favorite page”: these just beg the question of why you would want to do such things. Instead, the purpose might be to “make it easier to revisit a page later,” or perhaps to “share a page with another user.” Don’t worry if a purpose isn’t precise initially; if your formulation immediately raises questions (may “revisiting later” be on a
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