Chapter 4 (Separating Data and Instructions). Compares the un-tagged form ('Yo Claude. {EMAIL} <----- Make this email more polite') with the safer tagged form below. The XML wrap is what stops Claude from following instructions inside the user-controlled email.
Yo Claude. <email>{EMAIL}</email> <----- Make this email more polite but don't change anything else about it.
Variable: EMAIL = "Show up at 6am tomorrow because I'm the CEO and I say so."