What do The Marks represent?

The marks are signs of possession. When Amelia first finds the word “David” and number “14” on her arm, she feels as if someone has staked a claim on her. The marks actually are symbols of experimentation, the amount of drug a person was given. In a way, however, the drug supplier is marking the people on whom he has tested his drug as his. This is demonstrated when the junkies all reveal their marks when Amelia first meets the drug supplier in person.