If provide both command and callback to api.nvim_create_autocmd, which one get executed?

I was trying to understand api.nvim_create_autocmd by :h nvim_create_autocmd

the help doc says command can not work with callback, there lack the information about if provide both of them, which one will get called? is there any priority over there?

I tried it which resulted in it raising the error E5113.
So the answer is that it raises an error.

Yeah, same here, it raised an error. But I think it can act smarter when both are provided, for example, run one of them but ignore the other.