Often, "SG" refers to the Sega Genesis (or Sega Game 1000). Archives named like this are frequently found in private collections or ROM sets shared on community forums.
It could be a personal backup of documents, save games, or project files (e.g., "S.G." standing for a person's initials or a project like "Study Guide"). 2 SG.7z
Sometimes large files are split into multiple parts (e.g., 1 SG.7z, 2 SG.7z). Often, "SG" refers to the Sega Genesis (or Sega Game 1000)