The networked substrate Mordecai suggests Tagg transferred most of his mind into, leaving only a scrap of biology behind.
Assessing Carl's accusation that Epitome Tagg is a cyborg, Mordecai says it would make sense if Tagg were grafted into Louis's biomechanical heart or pulmonary system — he would need at least some biological form, but if he had transferred most of his brain to the Reaver net, that could work.
Mordecai calls it a wild theory, noting Tagg is one of the most famous CEOs in the galaxy and that would be an enormous secret to keep. Rosetta supports it, explaining that Reavers can isolate processing systems in any part of their bodies.
The distributed Reaver substrate that could hold Tagg's mind while his body is destroyed.