The configuration settings of a particular reminder determine the way it is sent. See Creating Reminders for details.
If you set up automatic reminder sendings and specify the delay time, it will do so if the server is configured to run this script. See Scheduled X-Cart Tasks for details.
If a reminder needs to be sent not to all but just some of the abandoned carts on your store's abandoned carts list, you must send it manually.
To send a reminder email to owners of specific carts:
Open the Abandoned Carts page of your store Admin area (Orders -> Abandoned Carts).
In the list, select the carts you wish to send the reminder message to using the respective checkbox in front of them.
The Remind selected customers button below the list becomes active.
Click Remind selected customers.
A popup titled Choose reminder appears:
In the popup, use the Reminder drop-down box to select the reminder message you wish to send and click the Send reminder button.
That's it! Your online store will send the reminder message to the owners of the selected carts, and the respective note will appear in the list.