(Capcom's primary storyline)
Magneto-optical discs are a storage medium which entered the market in 1985 as a higher capacity alternative to the CD-ROM, useful for storing computer data.
The Umbrella Corporation heavily utilised storage technologies in its computers, and used MO discs for operating terminals. One of the manufacturers of these discs was, in fact, an Umbrella subsidiary, which helped to keep the movement of their products quiet.
At the Arklay Laboratory, at least three drives were used in the facility, again as a security system. As access to the test subject containment cell was restricted, each department head was required to read their respective discs. A similar setup existed at the Umbrella Executive Training School, possibly installed during the 1998 refurbishment. This system required an MO disc be read to unlock doors to restricted areas, as well as the entrance.
Another purpose of MO discs was in bioweaponry. During the 1998 Raccoon City Destruction Incident, the Tyrant R was woken up from its comatose state when an MO disc relayed instructions to the computer monitoring him.