What exactly is in the LR catalog?

Question

I can think of image location references as well as references to the corresponding preview files and Lightroom proprietary image metadata are held in the catalog, but I doubt that develop settings are there because either xml files directly alongside the images are used for that or development settings are stored in the dng files themselves. And for presets etc. Lightroom uses files in the filesystem. So what exactly is in the catalog?
Andreas

Answer 1

dj_paige wrote:
Everything you do to your photos in Lightroom is in the catalog.
Furthermore, there are some organisational matters that the Catalog holds, which are wider in scope than any image taken in isolation. Most critically, the Catalog discriminates which image files you have chosen to import, and which you have not imported (or have since deliberately removed).
Each Catalog has some workflow settings of its own - so one Catalog may be set to write XMP out to the files automatically, and another not to do so. Of course, the Catalog conveniently stores the last view and workspace that you had for your library, including any filtering, stacking or custom ordering of the images.
Each catalog holds a list of keywords (regardless whether these have been used or not) each with nesting, synonym and other properties.
Each Catalog provides Collections, Smart Collections, Print Collections etc which have no existence outside that Catalog.
Each Catalog can maintain Publish setups, collections, smart folders, and ongoing change-tracking relationships with external image copies either locally or online.
The same for remembered print settings of each image, page setups, softproofing settings etc
The same for Virtual Copies along with all their text and develop metadata - these are treated within the Catalog (only), as fully functional images in their own right. But if you go to the folder on disk, there is nothing to see of these whatever.
Some people put quite a lot of effort into settings which are in fact available outside LR and its Catalog, because they are "shared" with ACR. This includes processing defaults, lens and camera calibration profiles. Also some LR settings saved via a given Catalog, are centrally stored by default, so available to other LR Catalogs: chiefly develop presets, import or export presets, and the like.
Edit: I forgot one of the most important: a History of the steps carried out, giving access to all the different states that the image has passed through.