KK is the reservations systems mainframe code for a confirmed reservation. "I" is for "Instant". Basically it allows a SE to get any economy seat (assuming positive space, ie: not an oversell) they want for an Aeroplan reservation, instead of being subject to the usual inventory constraints. The theory being, SE's spend enough $$ with AC, and are flying enough that they deserve to at least be able to use the rewards the AP program offers.

There are limits because obviously a SE could obtain millions of points with credit cards or similar, and use the points entirely on tickets that otherwise would only be available with full-Y inventory (ie: $$$$$$, $3000 for a transcon, for instance!). Depriving AC of that revenue.