Promos are usually used to promote an upcoming release by being played on the radio or reviewed by journalists. These items can be custom made or retail items with sticker or cut-outs. There are also promo discs which are entire radio broadcasts. Some promo items have other uses, like the Australian 4CD From The Muddy Banks Of The Wishkah promo box set (NIRV 1). Those who pre-ordered the album entered a competition where the promo box was the prize [1]. Some were also used to presell releases to retailers, like for example Tupelo test pressings [2], and the tapes described below.
Some cassettes (example) and videos (examples) were used to presell new releases to retailers, in addition to regular promotion. Rob Fellows explains: [3]
When I worked as a rep for MCA/ Universal I was part of a team that went around to record stores (mostly independent stores) pre-selling forthcoming releases. We would have promo tapes of new acts sent out to us to give out to the stores in order to get in-store play, or to get the staff excited about a new act, or a new release by an established act. In the early 90's we were only sent tapes, they were just a cheap way of sending out music. CD-Rs are used instead now, but we wouldn't have sent out CD-Rs on Nirvana. These tapes would also have been sent to journalists. They would not have had a catalog number. They were not really "releases" as such. The only things to get catalog numbers are official releases and promos. The tapes and the CD-Rs that we use today are not cataloged and are not what I would call a true promo. We just copy up as many as are needed by press, promotions and sales depts for use as I explained above.
Quite a lot of these tapes would have been run off, but I guess they are rare because, like me, most people would not have seen any value in them and would have given them out to the stores as part of their job and probably thrown away any leftover ones. Also you have to expect that the stores and journalists that were given these would have been bombarded with similar things for other acts from every record company in the UK, they were so common I guess most ended up in the trash.
Promo videos were used mainly to show to retail accounts in-store on their in-store TVs, and used in-house for internal presentations. Basically the same as the tapes. As they are not really promo items, they do not have catalog numbers. [3]