Similar to the "fives" rule on seats. However, with fives, you can only save your seat for five minutes. Calling squaters means that you can save your seat unless you sit down somewhere else. The moment your ass touches another seat, squaters is ended, and the first seat becomes available.
Bob: "shit my phone is ringing, but i dont want to lose my seat..."
gets up to answer phone, and someone steals his seat
Jack:"He should have called squaters"