Board Thread:Questions and Answers/@comment-49.176.4.79-20121217050806/@comment-969093-20121217083258

Simply means it will not take any damage from that element on that particular spot(s) in that one situation. You can look at it as attacking the monster with the same LS but then a Raw-only version of it. Like 1104 Raw + 300 Water will simply turn into 1104 Raw (not literally but it's just to illustrate).