A session is defined the same as it is within Shopify. The definition taken from this Shopify page is below:
"The number of sessions and the number of visitors are based on cookies. Cookies are small files that are stored on a shopper's device, such as a desktop or smartphone, when they visit your online store. One cookie identifies the device (the visitor). Another cookie keeps track of the length of the session.
A session ends after 30 minutes of no activity, and at midnight UTC. Because the same visitor can have multiple sessions, the number of sessions is usually higher than the number of visitors.
Example: A shopper views products in your store for 20 minutes, then returns to your store 2 hours later for another 10 minutes. Two sessions and one visitor are reported. If the shopper instead views your store for 5 minutes, leaves, and returns 10 minutes later, then one session and one visitor are reported.
If you want to view your online store and not have it count as a session in your acquisition reports, then on your desktop Shopify admin, click the eye icon—any other method of viewing your online store on desktop counts as a session in your reports.
If you view your online store from the Shopify app on iPhone or Android, then it counts as a session in your acquisition reports."
Rep AI tracks sessions very similarly to Shopify, even though there might be a slight discrepancy where Rep may actually count fewer sessions.
The discrepancy may be caused by the fact that Shopify ends sessions at midnight UTC, and Rep does that at midnight Eastern timezone.
Another cause for the discrepancy may be caused by the fact that Rep loads on the page later than Shopify and some visitors might have already left the website by then and therefore would not be tracked by Rep.