PSDT file is a Photoshop Document Template File. The difference to a normal . PSD file is that it opens as “untitled” and requires you to save it as a new file, preserving the original from overwriting.
If you have Photoshop files that you use as templates, consider using a Photoshop Template file. To create a Photoshop Template file simply add the letter “T” to the PSD file type. The PSDT file will open an instance of the original file, and you will never override the original.