Airaid reduction sleeve?

Hello all, I'm new to the forum and have come seeking knowledge. I've searched the forum and the internet and could not find any information on the 99-04's regarding this. I've recently installed an Airaid cai on my 2004 Mustang GT with an MRT offroad H pipe and Flowmaster American Thunder's and a few other mods. I'm having it tuned by Bama and they are saying to make sure to remove the "reduction sleeve" on the Airaid for the tune to work properly. I have no idea what they are talking about. I thought maybe the "velocity stack" but then I wouldn't have anything to install the filter onto. Would someone please enlighten me as to what they are talking about? Thank You