A sequential numbering system developed by Trent as a way of keeping track of all releases produced under the name "Nine Inch Nails". For instance, halo one is the Down In It single, the first item ever released by this band. Pretty Hate Machine, the second item released, would be halo two (and so on and so forth). These halo numbers are usually printed somewhere on the CD/Vinyl sleeve or jacket.
Many of these releases are now out of print. Halo 1, Halo 2, Halo 3 and Halo 4 are out of print due to Reznor's conflict with his former label TVT Records. Halo 13 and Halo 15 are also out of print. When any of these will be back in print is unknown as of the time of writing of this article.