If anyone is interested, I can explain some of the technical underpinning with keywords. Most image file formats have reserved space for metadata, which is data about the file (keywords, ratings, develop settings, copyright, and so on.) Some formats, like RAW files, typically have a linked sidecar file which goes with the image and contains that metadata. A number of different "namespaces" are defined to store metadata, each with specific fields and formatting required. These namespaces can overlap so that metadata is often duplicated inside a file. Keywords can be stored in the "DC" namespace with a "Subject" tag but also in the (apparently undocumented) "LR" namespace with a "hierarchicalSubject" tag. Ideally, keywords would be saved to both namespaces in, say, a JPEG file. Most graphics software reads the DC namespace. I've written some scripts for Bridge to assign keywords and I write keywords to both namespaces. I don't know how mobile apps or Lightroom CC operate but presumably, using both namespaces would mean that keywords were truly portable. Note that metadata can be written to files AND/OR to a catalog like Lightroom uses. If files are not updated and changes are only written to a catalog, its up to the developers to export that data as needed. If you run File Info on an image using Photoshop or Bridge and look at the Raw Data, you'll be able to see the structure of the metadata and what is present or not. Finally, there are both third-party programs such as EXIFTool and add-ons for Adobe products that let you work in depth with metadata including keywords.
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