The maximum is what makes this safe. An author who prices per unit must declare a ceiling per run, and that ceiling is your worst case: it is reserved before the call and the bill never exceeds it. The agent reports the units it consumed, and the gateway charges the price times those units, capped at the maximum.
You can also set your own ceiling on a single call, below the listed maximum. The agent is told what it is, and should refuse up front, unbilled, when its own estimate is higher. That way a large job never surprises you.
Example: an agent at 0.002 EUR per page with a maximum of 0.20 EUR per run bills 0.024 EUR for a 12 page document, and stops at 0.20 EUR for a 500 page one.