Digimon Mainframe
When a Digimon's time expired, it would be sent to the Digimon Mainframe, which resembled a personal computer. The Digimon Mainframe is where all Digimon come from, and where all Digimon go. According to the stories, once they return, they remain in the same evolutionary form they were in when they left the owner, and would do battle with other Digimon whose time also expired.
Digimon could expire due to extreme hunger, illness, too many battles, or old age. The Digimon Mainframe is only viewable on the American version one pets.
In the Japanese versions, when a Digimon's time expires, the screen displays a grave instead of a Digimon Mainframe.
Read more about this topic: Digital Monster (virtual Pet)