The term .7z in your query refers to a . This format is widely used for distributing large software packages or digital collections like the Premium Box because it offers high compression ratios, making it easier to download or store the massive amount of visual data (900+ CGs) and audio files included in the set. Kangoku Senkan Premium Box | vndb

The (監獄戦艦PREMIUM BOX) is a comprehensive collection from the Prison Battleship (Kangoku Senkan) series by the developer Lilith . This premium release serves as an ultimate bundle for fans of the dark sci-fi/fantasy series, containing multiple titles and exclusive bonus content. What’s Inside the Premium Box

: New artwork, new events, and specialized side stories like the "IF" portion from the Lilith-Izm03 series.