What's the difference between category price and Default price?

Settings > Categories

The Default price option in Settings > Categories > Default price allows you to easily set a default price across all application categories, without having to set individual pricing for each.

The Default crossed-out price setting is a simple visual indicator which indicates that your current default price offers better value.

When you set individual pricing for a category, it overrides the default price and the default crossed-out price.

IMPORTANT: Setting a default price changes the price for any existing categories that don’t have their own price set. Keep in mind that changing the Default price does not affect existing registrations, only registrations that are made consequently.