For Microsoft 365 Enterprise clients
"To help protect you, Office opens files from potentially unsafe locations in Application Guard, a secure container that's isolated from the device through hardware-based virtualization. When Office opens files in Application Guard, you can securely read, edit, print, and save those files without having to re-open files outside the container.
If you believe the file is safe, you can choose to open the file outside Application Guard. If your administrator has enabled Safe Documents, the file will be verified against the Microsoft Defender Advanced Threat Protection (MDATP) service to determine if it's malicious before it's opened outside Application Guard."
"To help protect you, Office opens files from potentially unsafe locations in Application Guard, a secure container that's isolated from the device through hardware-based virtualization. When Office opens files in Application Guard, you can securely read, edit, print, and save those files without having to re-open files outside the container.
If you believe the file is safe, you can choose to open the file outside Application Guard. If your administrator has enabled Safe Documents, the file will be verified against the Microsoft Defender Advanced Threat Protection (MDATP) service to determine if it's malicious before it's opened outside Application Guard."
Application Guard for Office - Microsoft Support
Application Guard helps protect your PC from potentially malicious files.
support.office.com
Safe Documents - Microsoft Support
support.office.com