Excellent question, and an important one to understand. The Performance Creator which allows the performer on stage to quickly create a Split or a Layer, on-the-fly, without hitting EDIT. The other method of creating (more complex) Performance region arrangements is done by dropping into EDIT.

A SPLIT is defined by one point called the SPLIT POINT. This is one key, all notes including the split point note and above are considered the Upper, all notes below this key are considered the Lower. But significantly if the [SPLIT] light is lit there is only one split point and a Part is one or the other, or both.

When you drop into EDIT and use the NOTE LIMIT parameters per PART (SPLIT button is not lit), you can create a setup where each of the four Parts could have their own note region. These NOTE LIMIT settings are ignored if the SPLIT button is activated.

So when Yamaha uses the word "split", it is one cut that yields two sections: an upper and a lower.
When you do not use the Split function but rather use the NOTE LIMIT you can set each Part to its own region, even overlapping regions if you wish.

Thanks for the explanation. I have still a lot to experiment 🙂