"Send REST API notifications by email" is an option that you can configure to send an email notification after users perform some actions on kintone using REST API.

The setting can be configured under the Email notifications section on the Feature activation screen.
Activating features
Settings to activate email notifications

Notifications for REST API actions include the following notifications:

Example:

• A General notification sent when a record is added through REST API
• A notification sent when an assignee is updated through REST API
• A Per Record notification sent when a record is updated through REST API

Note that Reminder notifications are not triggered by API actions. Reminder notification emails will be sent regardless of the REST API notification setting. For example, if you disable Send REST API notifications by email and then use REST API to add records, a Reminder notification email is still sent when the reminder condition is met.

If you use REST API to automate actions on kintone and also enable "Send REST API notifications by email", you may receive a large number of notifications by email. To avoid receiving numerous email notifications, disable the "Send REST API notifications by email" option and check the notifications on kintone.
Checking notifications