Segment

A segment is a defined subset of your data, scoped to users, sessions, or events, that you apply to focus a report or exploration on just that slice.

You build a segment from conditions on dimensions, metrics, and events, and pick its scope. A user segment carries everything that user did; a session segment isolates matching sessions; an event segment isolates matching events. In Explorations you drag a segment in to compare it against others, and you can turn a user segment into an audience for ads.

You define a user segment as anyone who triggered add_to_cart but never purchase in the period. Apply it to a path exploration and you see exactly where these near-buyers went instead of checking out, which a whole-property view would drown out.

Scope changes the answer, and people pick the wrong one. A session-scoped segment and a user-scoped segment built from the same condition return different numbers, because one asks "did this happen in the session" and the other asks "did this user ever do it." Decide whether your question is about visits or people before you build, or the count will mislead you.

Segment scope is the setting that quietly decides whether your number is right.