But I would be using IR film already... I'm confused.

I'm also kind of a newbie with infrared but I'll try a very basic explanation. Although you are using infrared film you need a filter that cuts out all visible light otherwise the infrared light will be overpowered by it and you won't get the proper infrared effect (just a normal lighting effect). That's why you need an opaque filter -- it will cut out the visible light and only let the infrared through (although how much depends on the filter and film used). With HIE you could use a red filter (25 or 29) and still get a good infrared look, but that is not the case with the remaining infrared films. The RM72 is a good choice because (I think) you can use it with all of the remaining infrared films.