ultisnips for my snippets in my Vim (gVim on Windows). I don't know the meaning of
[S] in this picture.
This picture shows part of my snippets in vim.
I believe it provides you hint about the source of the suggestion:
[A] the word is used in this buffer
[B] the word is used in another buffer
[F] the suggestion comes from the file system
[S] the suggestion comes from the snippets list
[LS] the suggestion is provided by the Language Server
You can get the full list of sources using the following command:
More information in the Coc documentation
For Language Server suggestions you get an additional information about the type of the suggestion: