Devices that enroll via Open Enrollment will apply this blueprint if the user-selected blueprint is unavailable for any reason. Devices that automatically enroll via DEP (if enabled) will apply this blueprint if one is not preassigned.
You can set your Default Enrollment Blueprint at any time from the Device Settings section of Jamf Now. Click on the Edit Settings link in the side navigation, then choose Devices. You can select your Default Enrollment Blueprint from any Blueprints in your Jamf Now account.