Stiyl Magnus has a bar code-like tatoo behind his eye. Why does he have it? Does it have a special meaning related to magicians?
As mentioned on this fandom-
Stiyl promised that he will always protect Index whatever it takes, though it is unknown at which point in time he made this promise. Stiyl also received his trademark barcode tattoo, ear piercing and rings after Index had her memories erased.
Apart from this I don't think any special significance is ever mentioned as it would have surely been reported on the fandom and other sites.