Logicola

Halo numbers are a system by which official Nine Inch Nails releases are chonologically ordered. They correspond to the sequence in which the releases were made, and are named by the word “Halo”, which precedes the number on the release. For example, the fifth Nine Inch Nails release, Broken, is identified with the phrase “Halo Five”. Halo numbers are sometimes modified for alternate versions of a release, such as the multiple releases of The Downward Spiral. Promotional-only releases do not have their own Halo numbers, although the promotional singles for “Piggy” and “Hurt” were both labeled as “Halo Ten,” a title later officially used for Further Down the Spiral. A U.S. promo for “Into the Void” is mislabeled as Halo 16 because this number actually belongs to the Things Falling Apart remix album. A bootleg titled 'Suck' containing the full version of Suck and a live version of Supernaut is typically referred to as Halo 0.

via Nine Inch Nails discography – Wikipedia, the free encyclopedia.