Frankly, I don't know. At all.

The reason has to do with the differences between Casual and Advanced than with the actual term "High-Casual". 

Usually, I see "High-Casual" when the RP requires [b]longer[/b] posts, but I don't see why that makes it more advanced. I mean, I'm an advanced RPer, and quantity actually means less in Advanced than it does in Casual. That isn't to say that posts in Advanced are often short - they usually aren't; they are usually quite lengthy indeed.

Simply, I don't believe length makes it Casual or Advanced. I think its the quality that decides whether it is Casual or Advanced because I have honestly read whole walls of textual crap in advanced but then also five sentences of written glory. I reference to the RP "Blight" that I'm in right now, which has had a couple posts that are pretty short that are from the GM himself, yet the whole of the RP seems to fit the standard that has been created for Advanced RPs.

Therefore, I'm not sure why people use "High-Casual". I mean, if we think about it critically, it probably isn't one, simple reason. It can be a multitude of reasons. Maybe they wanted to reach a specific pool of people? Maybe even a smaller or larger pool? Perhaps they have their own beliefs as to what constitutes an Advanced or a Casual RP and have decided that their RP is between the two? Maybe they just like flinging the word around? Who knows? 

I just don't think that this topic is simple enough to answer correctly.