I'm going through my vim dotfiles to tidy them up. I've noticed that through time I've added various filetype specific settings in various inconsistent ways. Let's suppose I'm customizing for Python:
au BufRead,BufNewFile *.py (do something). I don't like this because some Python files might not have the .py termination.
au FileType python (do something). This seems a better option because it doesn't depend on the file having the .py termination. The drawback is that Vim doesn't know about some filetypes. I can make Vim recognize additional filetypes, but I also have various inconsistent ways of doing it: a .vim/filetype.vim file, another in .vim/after/filetype.vim and various set filetype commands in .vimrc.
Add a .vim/ftplugin/python.vim file with filetype specific settings. I understand the $VIMRUNTIME/ftplugin/python.vim can override whatever settings I make here. One problem is that I'm not sure how this interacts with .vim/filetype.vim and .vim/after/filetype.vim.
Add a .vim/after/ftplugin/python.vim. I understand that this is loaded after $VIMRUNTIME/ftplugin/python.vim so it can overwrite settings from there. As in the previous method I'm not sure how it interacts with the filetype.vim files.
So I have at least four ways of doing this, not mentioning syntax files and filetype-specific plugins. It seems to me the best way to do this is to put my filetype specific settings in after/ftplugin so they don't get overwritten, and filetypes.vim in after for the same reason.
However, before I proceed I'd like to ask if anyone has suggestions about the best way to deal with filetype specific settings.
I store my Python-specific settings is in $HOME/.vim/ftplugin/python.vim, since I do nothing to conflict with $VIMRUNTIME/ftplugin/python.vim.
Keeping these settings in an ftplugin file keeps my .vimrc nice and generic and easier to maintain (it's still pretty big after over ten years of Vim usage).
If you want to overrule what the ftplugins with your Vim distribution set up, then $HOME/.vim/after/ftplugin/python.vim is what you want, as it is read after these.
Similarly $HOME/.vim/filetype.vim will be sourced before $VIMRUNTIME/filetype.vim and that in turn will be sourced before $HOME/.vim/after/filetype.vim.
Invoking :scriptnames will list all sourced script names, in the order they were first sourced.
:help filetype provides pretty comprehensive information on all of this.