Hi Veronica, my name is Alvaro and I'll be glad to help you today.
The external turn in shows up whenever a student uses an external account (e.g. personal account) outside of your organization to pass an assignment.
Here's the complete article regarding this access. If this is something that you want to be disabled in your organization, you may contact your IT administrator to discuss the changes internally as external access is enabled by default for all domains.
https://learn.microsoft.com/en-us/microsoftteam...
I hope this answers all your confusion about this matter. If you have other questions, just let me know. Have a nice day!