IBM 3590 - Manual (Page 71)

Notes:
1. A sensing device detects any cartridges positioned beyond the import
position and prevents operation of the cartridge transport.
2. Cartridges can be loaded or unloaded from the magazine from either side
of the magazine before it is installed in the 3590. All cartridges must be
loaded with the label side up and the leader block towards the machine
side of the magazine.
When the drive ejects a cartridge, in all modes except Random mode, a
cartridge is placed in the export position. The cartridge can be removed,
transferred to storage, or transferred to a different tape subsystem. The first
detent holds the cartridge. In Random mode, the tape subsystem always
places cartridges in the Import position in the magazine.
Chapter 4. Automatic Cartridge Facility (ACF) and Magazine 51