What's the difference between to_user_id and in_reply_to_user_id?


I’m trying to figure out which tweets are replies to a previous tweet using the search api. I thought the in_reply_to_status_id field and in_reply_to_user_id field would give me the id of the tweet that is being replied to and the author of that previous tweet. But there’s also a to_user_id field that doesn’t contain the same info as the in_reply_to_user_id. What’s the difference between the user in the to_user_id field and the one in the in_reply_to_user_id field? For many tweets, the in_reply_to_user_id field is null while the to_user_id field has an id.