That's the new offset. It will usually repoint the Pokémon/trainer picture and delete the old one (and change the name if you check that, too). As long as you check "Insert as pokémon/trainer" you're alright. You can either let agi find a free offset for you or determine one of your liking by entering it by hand (if you do that, you have to find a free spot yourself in the rom that has enough free bytes for the newly compressed data, it says this on the preview frame).
It is then that you choose pokémon number and everything else and set it ready to go

cYa,

Tauwasser