CD-R
CD-Rs are commonly used for promotional-only purposes as they are inexpensive to produce. Most are issued in a plastic slip case with a paper insert, although some have a more elaborate case, such as the double slip-case "Supermassive Black Hole" promo with a scratch-away panel.
Occasionally, the data on promotional CD-Rs is encrypted, such as the Absolution promo, so that the tracks cannot be leaked prior to the release date, whilst others are watermarked, so that the leaked songs can be traced. Some CD-R promos are unique, made for very specific purposes within the music industry, whilst others are mass produced and are relatively easily available from sites such as eBay.
They are often referred to as "acetates", a name inherited from the lower-quality acetate vinyl test-pressings used before the advent of digital formats.