What is "alternate takeoff" in this PT6A performance table?
Alternate Take-off is a reduced power option feature, similar to flex thrust on turbofans.  The aircraft manufacturer will provide performance data applicable to the reduced power setting that can be used when circumstances permit.  It's all about reducing wear and tear by not running the engine as hard if it's not essential.
The Max Continuous power setting is intended for emergencies (mostly single engine climb, or both engine climb in some marginal situations, or maintaining altitude single engine where you are one engine and high terrain makes it imperative to not descend) and as such is rarely used, so there is a higher limit to allow even a tired engine to make Max Continuous HP in a critical situation.
The reduced AT ITT limit reflects the objective of limiting wear and tear with a power setting used regularly, whereas the MC limit reflect the balls-to-the-wall limit the engine can take for extended periods when the power is desperately needed.