In Stranger Things, the number tattooed on Eleven’s left forearm is 011, commonly shortened in trivia answers to 11. The marking identifies her as Test Subject Eleven at Hawkins National Laboratory.
The tattoo is shown early in the series, when Mike, Dustin and Lucas encounter the mysterious girl after she escapes from the laboratory. The boys learn her designation from the “011” marking, and Mike begins calling her Eleven, or “El.” Her birth name is Jane Ives, although she later becomes known as Jane Hopper.
The leading zero matters inside the story because it shows that Eleven was part of a numbered sequence of test subjects rather than simply being given the name “Eleven.” Other children in the program were identified by numbers as well, including One, the character later revealed as Henry Creel and Vecna. The tattoo is therefore both a visual clue to Eleven’s identity and evidence of the laboratory’s experimentation on children.