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It is most probably a setting allowing you to adjust the magnetic (instantaneous) trip current of the circuit breaker.
For a non-inductive load with minimal inrush current, you might only need 3-5 x In. For a transformer or motor load, the initial inrush may be 14 x In or more, so you would need to increase the setting on the breaker to avoid it tripping on startup.
 

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What does that mean on a thermal/ magnetic trip unit. ( breaker)
That would be on a electronic trip unit, how In is derived varies for different trip units so need more info to answer your questions.
 

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If this is the KT molded case circuit breaker with a Thermal-Magnetic Trip Unit, then the In is instantaneous trip a multiple of the circuit breaker rating.

Depending on the circuit breaker rating that is the adjustable instantaneous

On a 350 amp CB this is adjustable from 1750-3500 amps 5-10 times the rating for Instantaneous trip..
 

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On cuttler-hammer/eaton breakers, In is your rating plug. You can get a 800 amp frame and have a 400A rating plug or a different size rating plug up to 800A. If you look on the rating plug for that example it will say In=400A. Your settings on the trip units are multiples of In. For example on a typical 11ARMLSIG you have a LSIG trip curve, meaning you have long short instantaneous and ground fault pick up. Your long delay pickup is a multiple of you In, so say you had In=800A and the long time pickup is set to 0.4. This would mean your long time pickup will be at 320A. Each part of the trip curve is related back to your In.

However with K frame molded case breaker you are probably referring to Im... KT represents K frame with Thermal Magnetic trip unit (KT). You can also get the K Frame with the seltronic trip unit, identified as KS. "The magnetic trip on each pole may be adjusted by turning the adjustment button with a screwdriver. Trip units are shipped from the factory with the buttons set in the low position. The magnetic trip (Im) is calibrated at the factory to trip at an rms alternating current within plus or minus 10 percent of the high setting indicated on the trip unit nameplate, and within plus or minus 20 percent of the low setting "
 
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