Reactions are short comments, consisting of just a couple of words or just an emoji - they were introduced to give our members the option to have a greater variety of comments on their tracks. If you do not leave a comment when reposting, you will often be given the option to leave a Reaction (to 'react') once you have left a rating:
A Reaction takes the form of a comment on SoundCloud itself, however in regard to our system a Reaction is distinct from a comment, therefore Reactions are not subject to the same checks or requirements as regular comments (minimum length, spam checks etc).