The last known modification date of the document’s on-disk representation.
NSDocument default file saving machinery uses this information to warn the user when the on-disk representation of an open document has been modified by something other than the current app.
Reading From and Writing to Files
var keepBackupFile: Bool
A Boolean value indicating whether the document keeps the backup files created before the document data is written to a file.
var isDraft: Bool
A Boolean value indicating whether the document is a draft that the user has not expressed an interest in keeping around.