A Layer is a group of one or more fields. A Layer contains all metadata of a Content Item, the so-called metadata profile. Examples: Basic Information (title, description), Product Information (product name, family, etc.).
The fields contained in a Layer become available when added to a content item. If the Layer is later removed, the fields it provided (and any assigned values) are also irreversibly removed. By adding and removing fields in Layers, users can define metadata profiles that are unique to each piece of content, and adjust those profiles over time. This is the basis for Adaptive Metadata™.
Layers are very important for the look and feel of your Picturepark in that they determine the structure of your data. Layers and the fields therein are also key for what you would like users to find via the simple search as well as the Channel filters you can create.