This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
I use the term "robotics" to refer to access to multiple removable volumes by a fewer number of drives without a person. This includes sequential stackers, as well as random access robotics. A stacker typically is capable of taking (literally) a stack of tapes and putting them into the drive one at a time, in order. No random access to specific tapes, as with a full-function autochanger. Stackers typically are limited to 8-10 cartridges, and are used by people whose backups have exceeded the size of one cartridge. In the larger media formats, such as D-1, D-2, Betacam, etc., the traditional manufacturers of broadcast autochangers, such as Asaca, Odetics, Sony, etc. have products that are easily adaptable to storage use. The August 1996 issue of Byte magazine has an article comparing 12 tape autochangers. It is a little misleading, not mentioning any of the truly large library systems, and only one midrange, whose capacity is quoted assuming DLT 7000 tape drives, which is never mentioned. In addition, much of their testing is more related to the drives than the autochangers.