That explains a lot of my confusion, I started digging and I'm using cssbundling-rails with tailwind, so I have tailwind.application.css, what I understood is I can't use import in tailwind.application.css and I should use tailwind in postcss or set a separate preprocessor altogether.   

So I have tried to install css:install:postcss rename my application.postcss.css to somethingelse.postcss.css my stylesheet link also requires my "somethingelse"
in package.json I have updated build:css command to include both preprocessorlike this 

"build:css": "postcss ./app/assets/stylesheets/somethingelse.postcss.css -o ./app/assets/builds/somethingelse.css --watch && tailwindcss -i ./app/assets/stylesheets/application.tailwind.css -o ./app/assets/builds/application.css"

Well it will compile both files, and both files are also included in the page, but the import statements are not working (in somethingelse.postcss.css @import "slim-select/dist/slimselect.css"; resolves to nothing) and also my now my profile which contains 

css: yarn build:css --watch

is unable to actually watch any of the files, I have to build css manually.

This is so confusing, the rails docs are missing documentation on this subject I have little to no idea what I should do.