Firstly, please mention the specific examination. In normal cases, if the caste slot is not mentioned then the candidate is treated as a general one. For example, If any SC/ST/OBC candidates misses out to give their respective caste, then they will be treated as a general candidate. So, as you have not opt for the rural reservation you will not get the advantage of that. So, I think your confusion is clear and if you have any queries feel free to ask . All The Best!!!