When the Engine calculates decision details for a display category on a product period, the Engine first asks the product period which rule class should be used for that display category (which is recorded on the product period as part of setting up your product).
The rule class specified on the product period/display category must ultimately extend from the ProductDecisionDetailsRuleSet.AbstractCase interface rule class. For ease of upgrades, it is recommended that your rule class extends the DefaultProductDecisionDetailsRuleSet.DefaultCase rule class which provides default implementations.
Your rule class will have one or more attributes annotated with Display or DisplaySubscreen, which is where the bulk of the implementation effort for your decision details work will lie.