Returning Customer Rate is a tricky metric. In most of the industry it is the same as Repeat Purchase Rate except Shopify considers it something differently.
In Shopify, the Returning Customer Rate is the measurement of customers who bought a second time in a given period of time.
While it's similar to Repeat Purchase Rate, Repeat Purchase Rate requires the initial order and second orders to be fully within the time. Returning Customer Rate on the other hand allows for the initial order to be outside the period of time (details here).
Returning Customer Rate can be useful when you're looking at a narrow date range of behavior data, especially if you compare it to your overall Repeat Purchase Rate. This will tell you if customers are ordering again more or less.
Returning Customer Rate has nothing to do with returning customers.
Eric Davis
Measure which customers you're retaining and which you're losing
In order to keep your best customers, you need visibility into what's going on with them. Repeat Customer Insights will help you track down where you're losing customers and how to better target new ones.