An E-sheet appears to be a piece of paper, but it has a digital interface which is touch-sensitive, and is networked. Users are capable of sending electronic communications with their e-sheets.
Zoe Graystone was in the middle of composing or sending an email with her e-sheet when the Train 23 was destroyed. ("Pilot") Sister Clarice Willow stole an e-sheet from a person on the street, allowing her to compose an email anonymously, which suggests that emails tracked back to individual e-sheets, and from there, their owners. ("Gravedancing")