Bitmap Brushes?

[Egg Bramhill]

Note the Jaggies ... A true vector brush wouldn't have this problem

Still confused. The inclusion of jaggies doesn't make the brush any more a non "true" vector brush, whatever that is. If the brush is a vector you should be able to scale it to any size and retain the resolution. You can select a brush created in XXP that's used vector shapes and convert the line to shapes and ungroup and select any of the shapes. They're still vectors.

My original question is where does this "bitmap brush" concept come from?

Jaggies are a product of the brush construction. To avoid them use smaller slices and transparencies within the creation vector shapes.