BootCamp creates a new partition (only one allowed) on the hard disk, which is no longer a part of the logical Fusion drive. Although it was intended for Windows installation, you can use it for anything you wish, such as large video or music libraries which you don't want under the Fusion control to keep them from ever occupying space on the SSD. You should be able to see the drive layout in "About My Mac" under the "Storage" tab. While it would be desirable to have Windows reside on the speedy SSD, the 128GB SSD that Apple provides with the Fusion option is too small to effectively do both.