What This Means
At 400, the Alaska presence is thin: one sailing, interior GTY, and a small fare component on Anthem of the Seas. It is a valid option, but not a strong one. The tier is mostly Mexico, so Alaska is more of a one-off than a real region strategy.
At 600, Alaska becomes much more usable. Anthem of the Seas appears again with southbound Hubbard Glacier sailings, and the tier starts to show a real Alaska pattern rather than a single outlier. By 800 and 1200, Alaska is firmly in the mix, with better cabin variety and stronger overall sailing quality.
If the question is where Alaska first appears, the answer is 400. If the question is where Alaska becomes practical enough to target, the answer is 600.