When you import a digicam file into the Aperture Library, you tell Aperture where to store it. If it is stored inside your Aperture Library package, it is referred to as "Managed". If it is stored outside your Aperture Library package, it is referred to as "Referenced". Aperture makes it easy to store your imported files on whichever directly-mounted storage device you want, and also easy to change the storage location at any time.
Once they are imported, your digicam files are called "Originals" in Aperture-speak. (In the manual, they also use the out-dated term "Master".)
For a quick review of the parts of Aperture, look here. I think you'll find it helpful.
Metadata is Image-specific. It remains with the Image, regardless of where you tell Aperture to store each Image's Original.
Lastly, to save space on your system drive, you might consider, as an alternative to referencing some (or all) of your Originals to an external drive, moving your Library to an external drive and leaving all your Originals managed. I have run Libraries off external drives for years.
Aperture program: on system drive
Your Library: either on system drive or on (fast) external drive
Your Originals: either in your Library (managed) or elsewhere (referenced), almost always on external drive.
