A SnapGene mirror file is the same as an alias in macOS or a shortcut in Windows. A mirror is a placeholder file that points to an original file in a SnapGene Project.
Under the hood a Vector NTI database has a flat structure which allows the same file to be "mirrored" across multiple subfolders. This structure is replicated when you import your VNTI database and create a new SnapGene Project folder. All original files are imported into top DNA or Protein folders. Each file is "mirrored" to the appropriate subfolder in the Project Folder.
In the example below the "original" ADCY7 DNA file is in the Project folder. A mirror file is located in the /DNA/Adapters subfolder.
When you select, view, edit and save a mirror file you are really viewing/editing the corresponding "original" file.
If Collections functionality (see Show Hidden Collections Functionality (SnapGene 7.0 and later)) is enabled when you import a Vector NTI database then it will import as a Collection. All original files are imported into the top folder or "Area" of a Collection for each file type. Each file is "mirrored" to the appropriate subfolder in the Collection.