An app-open event is broad, stable and often too weak. A push notification, accidental tap or authentication loop can produce presence without value. Retention becomes more informative when the return event corresponds to the reason a user would choose the product again.
Name the recurring promise
For a budgeting app, it may be reviewing transactions or completing a weekly plan. For a team product, value might require meaningful workspace activity rather than one member logging in. The event should be observable, consistently instrumented and available to the population under study.
Avoid an event that is too narrow
If the definition requires a rare advanced action, it may measure power-user retention while ignoring legitimate lighter use. Compare candidate events against qualitative knowledge and inspect how often each occurs across established users.
Keep diagnostic metrics nearby
Login, app open and session count can still help explain friction or cadence. They do not all need to become the headline definition. Document the chosen value event, alternatives considered and any product changes that altered its meaning.
A good return event is not universal. It is a transparent claim about recurring value that the product team is prepared to defend and revisit.