A Copy is a clone of a Sprite, Binome or other data-being. The process is known in the User-world as a system backup, making copies of data so that these additional copies may be used to restore the original in the event that it is ever lost or deleted.
The Principal Office or operating center of every single System in the Net holds a safe repository of every individual's PID codes, should anything happen to that person, if they were ever deleted or absent, then their retained copy is created next time the User, or System, schedules a backup or system restore, providing they register with the Principal Office of course.
The Copy is not an imitation however, it is a real living being and an exact duplicate of the original data-being it has replaced. Copies have the same personality, memories, thoughts and dreams of the original person.
However, in most cases a System cannot restore a person any further back than the most recent saved backup created by its User, as a consequence of this technicality, if the deceased original was corrupted or infected by a virus previously, their copy would be restored in that lesser-state as well.
Also, if the System is destroyed then all registered PID information on its populous is lost. If the original or copy survives on elsewhere in the Net then they are feasibly able to register with another system's Principal Office but never again from the former. When the Twin City was destroyed all PID information on its Sprite population was also lost, meaning it is impossible for Mainframe to restore these Sprites whenever it undergoes a system restore.
Copies cannot be created from Nulls, because Nulls retain the exact code from the Sprite or Binome it used to be, thus the System still recognizes they exist and does not make copies. When Mainframe was restored it was able to restore people who were deleted such as Number Eight and One Binome Jean-Luc, but not those who were nullified by Game Cubes. (End Prog)
Copy Girl being a Spectral formatted into a sprite, was able to utilize this ability and make multiple duplicates of herself to help defend the system from games, however these weren't proper system backups and so were temporary and not individual people. (Where No Sprite Has Gone Before)
Little Enzo is copy of Enzo Matrix when he was just "01". When Mainframe had crashed and was restored by the User, Matrix had forgotten to switch his icon from Game Sprite-mode back to normal. As result the system couldn't detect Enzo and created a copy of the small sprite. Though Dot Matrix, Enzo Matrix and Welman Matrix have come to treat him as family, he does sometimes cause confusion (Sacrifice). Later on Little Enzo eventually realizes he is a copy but doesn't mind as he has a larger family to look up to now. (My Two Bobs)
When the second Bob arrived in Mainframe, everyone especially Glitch-Bob was confused, because this Bob looked, acted and sounded like the original Guardian before he was shot into the Web. Strangely though this Bob claimed to have no memories after the events of the Web War, eventually he replaced Glitch-Bob making him believe he was actually a copy who went to live on in the Web. It wasn't until Dot's failed wedding did everyone realize this Bob was actually Megabyte as a Trojan-Horse virus in disguise, with Bob's stolen Guardian code. (Null Bot of the Bride)