Answer
When creating a custom Event, you explicitly choose which of the event's attributes should be made available to its actions — once you've selected and saved the relevant attributes, they appear as available substitution fields when you go on to create an action for that event, and you can then reference them directly as variables inside your action's SQL/PL/SQL block. This attribute-selection step during event creation is what exposes a given field's runtime value for use inside your custom logic.