Custom conversion
A custom conversion is a Meta measurement rule that counts events matching conditions you define - by URL, by event name or by event parameter - when a standard event does not describe your business action. You point the rule at an existing event stream; the events themselves do not change.
When you need one
Typical cases: counting purchases only above a certain value, separating leads from a specific landing page, or measuring an action for which your setup sends a generic event name. The custom conversion filters after the fact, so one event stream can feed several differently-scoped conversion rules.
Custom conversions and server-side events
Events delivered through the Conversions API participate in custom conversions the same way pixel events do. The rule's conditions must match fields your server payload actually carries — if a rule filters by a URL but your server events never include one, the rule stays empty. For affiliate flows without a website, the event mapping has to deliberately include the fields the custom conversion filters on.