Q: 3 TB drive replacement when it is a Fusion Drive

The 3 TB fusion drive in my late 2012 iMac has failed and is at the Apple Store for replacement under their warranty program.  The techs seem to know little about Fusion Drives--the one I spoke with didn't know what a Fusion Drive was.  How can I be sure that get back a Fusion Drive and not just a bog standard 3 TB DRIVE.