Libraries
A library in Aspect is where photos and collections are stored. This includes both, the photos themselves, as well as the catalog, along with its thumbnail and metadata caches.
Libraries vs. Catalogs
It is important to distinguish between how a photo management application that is based on a library concept behaves as opposed to one that just uses a catalog. In general, for library based applications, the images are contained in and owned by the library, whereas catalog based applications only store references to files and do not take ownership.
There two main reasons why Aspect is using a library based approach:
It allows to reduce the manual work required for organizing images. Most notably, instead of manually creating and renaming folders according to their date, events will fully automate this process. This ultimately allows the user to concentrate on working with the actual photos.
In order to provide a meaningful way to synchronize libraries between multiple devices, while still retaining an organized folder structure, it is necessary that Aspect has a semantic understanding of it. A purely catalog based solution simply cannot work well for this application. This is also the reason why applications like Lightroom CC or Apple Photos are using a library based approach, albeit using a closed approach for storing photos, instead of the transparent file system integration offered by Aspect.
The consequence of this approach is that it is not possible to add or remove individual photos from a library without also removing it phsically from the containing folder. While scanned folders offer a way to integrate photos, without moving or copying them into the library folder, the photos themselves are still considered to be owned by the library.