RAW files are not intended to store keywords inside and there is no designated room inside the file to do so. In order to overcome this limitation, some software vendors started to use so-called XMP sidecar files in conjunction with RAW files. An XMP sidecar file is a file that has the same name as a RAW file, but a different extension. This file is created and used to store metadata that is not possible to write into a RAW file. These files can store keywords as well. When you develop a RAW file with a mainstream software and apply keywords to it, there is a high probability that the keywords will be saved into XMP sidecar files.
Phototheca supports XMP sidecar files and reads their content on RAW files import. Therefore, keywords from those files are recognized by Phototheca and copied into its own index too. That means you can perform a search by those keywords.
Phototheca does not support writing into XMP sidecar files yet, so all changes in keywords made to RAW files have an effect inside Phototheca only. The functionality to write into XMP sidecar files will be added later this year.