Definition
Cross-selling is offering an existing customer a different product or add-on that complements what they already use: a scheduling tool offering payments, a store suggesting accessories. It differs from upselling, which moves a customer to a bigger version of the same product.
When it works
Cross-sells succeed when they solve the next problem the customer has, at the moment they have it. Product data shows that moment: the action that usually comes before a need for the add-on.
How to measure it
Track when the offer is shown and when it's accepted, as events. Compare acceptance and retention between customers who took the add-on and those who didn't, so you know it adds value rather than just revenue.