If a subscriber receives an email campaign, clicks on one of the links in the campaign, and makes a purchase within the next 7 days (the default duration), this purchase is counted as a conversion.
In this sense, conversion rate indicates the percentage of subscribers among your overall campaign recipients that perform the desired action after receiving your email campaign. It is calculated as (# conversions / # delivered emails) x 100.
The default goal of Insider's Email product is purchase. When a user makes a purchase, that is counted as a conversion. The overall revenue of the email product is tracked based on the last click attribution model.
For example, a user clicks on a product with a price of 10 USD in 5 different campaigns, and purchases it. Each campaign would have a revenue of 10 USD based on this purchase. However, the overall analytics unifies the revenue to not duplicate the data. That's why, only the purchase based on the last-click attribution would be recorded, and the revenue for this product would only be 10 USD in the overall analytics.