If you flag a post, you will see a choice called "rude or abusive", what is difference between "rude" and "abusive"? They seem the same.
Being "rude" is more of a passive action. If you walk by someone without greeting him or her, that would be rude.
Being "abusive" is being active in causing offense. If you push or bump into someone (intentionally), that would be being abusive.
In terms of flagging a post several actions, or reactions, tend to be grouped together. In this example "rude or abusive". In this view "rude" can be an action that induces a reaction whereas "abusive" is a direct action with intent (or uses inappropriate words or statements).
rude: "your post is dumb"
abusive: "no one with a brain would post something so dumb".