Why
A line, drawn in advance.
AI systems now answer questions and take actions for people with very different rights. The people who answer for them need to know where the line is, and whether the system stayed on the right side of it.
Plimsoll SI draws that line for each system: a limit committed before the test starts, measured exactly, and sealed so anyone can check it.
A seal drawn from one run's own record: change a byte and it no longer matches.