A Catalog can store single image files, Album, Album Intelligenti, Progetti, and Gruppi.
Album. Put an image into several Albums without creating copies or using more hard disk space. This saves hard disk space and ensures easier organization. Editing an image in one Album will, of course, be reflected in all other Albums which contain the same image. Note that when you place one variant of an image into an Album, all other variants of this image will be placed there as well.
Album Intelligente. Those are similar to Albums, however, the images are added automatically there based on the correspondence to the search criteria you set. You can always adjust the search criteria for your Album Intelligente (see the article Edit a Smart Album). Unlike Album, only the variant that fully matches the chosen criteria is added to the Album Intelligente, while other variants of the image are not added in case they do not match the search preferences.
Progetto. Group your Album into Proggetti, search and filter images within a Progetto. A Progetto will limit the search scope of, for example, Album Intelligente within it. A Progetto cannot contain other Progetti.
Gruppo. A Gruppo can contain other Gruppi, Progetti, Album, etc. - it is the simplest way to group items. An Album Intelligente located within a Gruppo searches for the files located outside the Gruppo. Meanwhile, a Progetto will limit the search scope of, for example, Album Intelligente within it and cannot contain other Progetti.