Also known as Priority Rewards.

Access to more seats using Aeroplan miles. For SEs, it's more seats at the regular Aeroplan fixed mileage price, both in Y and in J. In Y, it's simply subject to the flight not being oversold. In J, if there's R space, you can redeem Aeroplan points for the seat.

For non-SEs, the benefit is less valuable as it only applies to Y and costs 2x the regular fixed mileage level. In my experience, Market Fare is almost inevitably cheaper, but perhaps there might be times where a flight is nearly sold out and it might be worthwhile.

Used correctly, this can be a very valuable benefit, especially if you have a lot of miles that you rack up from credit cards.

IKK is limited to 10 per year though, so use them wisely.