Whenever you start marketing to customer segments you should use different UTM or other tracking codes to be able to track each campaign separately.
Some systems like Repeat Customer Insights can work out their purchases without a tracking code but many systems cannot and could attribute those sales as new customer sales. Especially if it's been awhile since they've visited your store or have switched devices (i.e. cookies can get lost or expire).
Adding a UTM code is easy and is used automatically by Google Analytics to give proper attribution to your segmenting campaign. Other analytics systems might use other codes but I've seen quite a few support Google's UTM anyways.
Compare how your sales perform
Comparing sales channel performance is vital to understanding where to invest your resources. Repeat Customer Insights will analyze the channels that send you customers so you can directly compare.