Room for obstructionEdit
A boat may hail for ROOM if she has overlap with another boat and when there is danger that the space between an obstruction and the other boat is insufficient for her to safely sail her proper course. Sometimes you may see WATER or LAND used in chat however, ROOM is the term used in the RRS and in a real race your protest may be denied if you use the wrong hail. See also rule 19.
Room to tack (RTT) at an obstructionEdit
This is best described by rule 20 and the commentary found at that article.