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
Optimize your promotion timing to save money and attention
Repeat Customer Insights will analyze a ton of customer behavior data for you, including their buying cycles.
If you knew exactly when the majority of your customers were ready to buy again, you can increase your orders and profit just by tweaking your message timing.