If you want to provide a discount when people enter multiple categories at your event, use the Multi-Entry Discount feature in Event Setup.
You can offer a fixed rate on additional races or categories (for example, each additional race costs $10) or a standard discount on additional races (for example, deduct $5 from each additional race). After choosing the type of discount, enter the amount of the discount, and click "save" at the bottom.
Want to automatically register people for additional races in your event or series based on their initial category selection? Use the Omnium/Series feature.
Want to provide a discount to people registering for multiple events in a series? Use the Multi Events Coupon feature.