Categories of life

The categories of life was a system used to classify people on Earth after Miracle Day, in which people stopped dying. People were taken to overflow camps with Category Is being secretly taken to modules and burned alive, since this seemed the only way Humans could die after the Miracle had come into effect. Since the formerly immortal Jack Harkness became mortal after Miracle Day, Esther Drummond joked that he was "Category Jack". The Categories were arbiritarily applied, as Category Is could become Category IIs if they began to heal, such as Rex Matheson who should have been killed but then healed leaving a nonfatal wound.

Category I
Those people designated as Category I were brain dead or otherwise unable to think or function. In the overflow camps, Category I patients were designated with a red clothes peg and intended to be sent to the modules to be burned.

Category II
Category II contained those who had non-fatal injuries. In the overflow camps, Category II patients were identified with a blue peg.

Category III
Category III referred to anyone who was healthy and so contained everyone not in Categories I or II. (TW: The Categories of Life)

"Category Jack"
Esther Drummond referred to Captain Jack Harkness as "Category Jack" due to his becoming mortal as opposed to the rest of the planet's immortality.