Hi Pattie,
Thanks for the question! The reason the rule is represented that way is to to attempt to make it as clear as possible. So, let's walk through some different representations and see what's good or bad about each.
Let's start with R
N. The problem I see with this representation is that it's not entirely clear what it means in a Linear game. In a Grouping game it would make immediate sense, but it could be confusing under time pressure. If it is meant to mean that whenever R appears that N appears somewhere in proximity, then to make it clearer, I'd add in exactly where N appears in proximity to R. And that's what we did in making the rule R
NR. With that representation, we know that whenever you see R, it must be in an NR formation. That helps clarify the exact nature of the rule and hopefully make it very easy to use visually.
One question that often comes up is why the rule isn't just represented with an NR block. That's because this rule is all about what happens when R is present; N is ok being alone, but R isn't.
Please let me know if that helps. Thanks!