AIUI, a dictionary is built during compression to specify the heuristics of a particular dataset and belongs to that specific dataset only. For example, it could be a ranking of the most frequent 10 symbols in the compressed file. That will be different for every input file.
That could be different for every input file, but it doesn't have to be. It could also be a fixed dictionary. For example, ZLIB allows for a user-defined dictionary [1].
In this case, I'd consider the LLM to be a fixed dictionary of sorts. A very large, fixed dictionary with probabilistic return values.
Admittedly, I don’t think it is common, but I think there was a project a few years ago (Google?) that tried to compress HTML using at least a partially fixed dictionary.
Nowadays though, it’s apparently still something that’s being tried. Chrome now supports shared dictionaries for Zstd and Brotli. One idea being, you would likely benefit from having a shared dictionary used to decompress multiple artifacts for a site. But, you many not want everything compressed all together, so this way you get the compression benefit, but can have those artifacts split into different files.
