You won't find these releases at your local music store.
A promotional recording, or promo, is an audio or video recording distributed for free, usually in order to promote a recording that is or soon will be commercially available. Promos are normally sent directly to broadcasters, such as music radio and televisionstations, and to tastemakers, such as DJs and music journalists, in advance of the release of commercial editions, in the hope that airplay, reviews, and other forms of exposure will result and stimulate the public's interest in the commercial release.
Promos are often distributed in plain packaging, without the text or artwork that appears on the commercial version. Typically a promo is marked with some variation of the following text: "Licensed for promotional use only. Sale is prohibited." It may also state that the promo is still the property of the distributor and is to be "returned upon demand." However, it is not illegal to sell promotional recordings, and recalls of promos are extremely rare and unenforced.
Because promos are produced in smaller quantity than releases made available to the general public, they are sometimes considered valuable collectors' items. They are never intended for sale in record stores.
Record Collector - September 1996