From ABB Operation Manual TPS57-F32
Operation above the indicated values $n_{B_{max}}$, $t_{B_{max}}$ can considerably shorten the recommended replacement intervals. In such cases, we recommend that you contact the nearest official ABB Turbocharging service station.
$n_{M_{max}}$ and $t_{M_{max}}$ normally apply only when running at overload (110%) during trials on the engine test bed. These limits can also be permitted during operation for special applications. Operation above $n_{M_{max}}$ and $t_{M_{max}}$ is not permitted.
$B_{max}$ is normal, not to be exceeded, operational limits, while $M_{max}$ are 110% one time, not to be exceeded, commisioning limits for the turbocharger (TC). As in: is everything working correctly if the TC is subjected to 110% of full-load.
From MAN B&W S90ME-C8-TII 2-stroke Diesel Engine.
Minimum delivery test
The minimum delivery test, EoD: 4 14 001, involves: • Starting and manoeuvring test at no load • Load test Engine to be started and run up to 50% of Specified MCR (M) in 1 hour
Followed by: • 0.50 hour running at 25% of specified MCR • 0.50 hour running at 50% of specified MCR • 0.50 hour running at 75% of specified MCR • 1.00 hour running at 100% of specified MCR • 0.50 hour running at 110% of specified MCR
During the commisioning process for large marine diesels, the diesel is run at 110% of Manufacturers Continuous Rating (MCR) for 30 minutes. The diesel and it's components must not exceed specific maximums, which can be tested. In the case of the TC, ABB states that frequency and temperature should be below these $M_{max}$ limits ($t_{M_{max}} = 650°C$).