*** Please note that Aperture XMP currently requires 32-bit mode and is not compatible with Aperture 3.3 or higher ***
Aperture XMP provides a way for Aperture users to export XMP metadata for other programs, such as Adobe Photoshop, to read. For referenced master files, you have the option to just write sidecar files automatically in the same place as your master image. For exported versions, Aperture XMP also allows you to embed the XMP data into your image.
Aperture XMP can be purchased for $9.95