For people who want to set <Tab> behavior more flexible

[xvzc]

" Use <C-j> for jump to next placeholder, it's default of coc.nvim
let g:coc_snippet_next = ''

" Use <C-k> for jump to previous placeholder, it's default of coc.nvim
let g:coc_snippet_prev = ''

" Use ctrl - space to trigger completion.
if has('nvim')
  inoremap <silent><expr> <C-space> coc#refresh()
else
  inoremap <silent><expr> <c-@> coc#refresh()
endif

inoremap <silent><nowait><expr> <Tab>
      \ pumvisible() ? coc#_select_confirm() :
      \ coc#jumpable() ? "\<C-R>=coc#rpc#request('snippetNext', [])<cr>" :
      \ coc#expandable() ? "\<C-R>=coc#rpc#request('doKeymap', ['snippets-expand',''])<CR>" :
      \ <SID>check_back_space() ? "\<Tab>" :
      \ coc#refresh()

inoremap <silent><expr> <S-TAB>
      \ coc#jumpable() ? "\<C-R>=coc#rpc#request('snippetPrev', [])<cr>" :
      \ "\<C-d>"

snoremap <buffer><nowait><silent><TAB> <Esc>:call coc#rpc#request('snippetNext', [])<cr>
snoremap <buffer><nowait><silent><S-TAB> <Esc>:call coc#rpc#request('snippetPrev', [])<cr>

" functions
" tab behavior
function! s:check_back_space() abort
  let col = col('.') - 1
  return !col || getline('.')[col - 1]  =~# '\s'
endfunction

I found that the original options are kind of restricted to customize, so i did a simple trick which is setting coc_snippet_next, prev keys to nothing. And then, we can do whatever we want like on inoremap <silent><nowait><exprt ... line, with given functions in coc.nvim

[chemzqm]

The problem is you may want to expand snippet instead of jump placeholder in some cases, so I don't like the idea to configure <tab> for too many things.

[xvzc]

Yeah right, but i set coc#_select_confirm() when it's pumvisible. I just wanted to share what i did for some people who may wanna do some similar things with this. Thanks for giving your opinion anyway.