What are the four Product Rule types in CPQ and when is each evaluated?

Product Rules (SBQQ__ProductRule__c) enforce configurator logic based on combinations of products or field values. The four types are:

1. Validation Rule: Prevents the user from saving the quote if conditions are not met. Displays an error message. Evaluated on save. Use case: block saving if a subscription product is added without a support option.
2. Alert Rule: Warns the user with a message but allows them to proceed. Evaluated on save or on change. Use case: notify a rep that a discount exceeds the standard threshold.
3. Selection Rule: Automatically adds, removes, hides, or disables options when conditions are met. Evaluated when a product is added or a quantity/field changes. Use case: auto-add a required connector product when a specific integration is selected.
4. Filter Rule: Filters the products shown in the add products search or catalog based on conditions. Evaluated when the product catalog is opened. Use case: show only products compatible with the selected edition. Each rule uses Product Conditions (what triggers the rule) and Product Actions (what happens).