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 Collection.
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 as a SnapGene Collection.
When you import a Vector NTI database into SnapGene 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.
In the above example the imported vector NTI database had 5 DNA sequences which, when imported, are "mirrored" in 3 subfolders.
The 5 original sequences are imported into the top "DNA" Area (selected and highlighted blue), mirror files pointing to each original file are added in the 3 subfolders.
When you select, view, edit and save a mirror file you are really viewing/editing the corresponding "original" file.