There are flat, angled, block and trim brushes to name a few.
Flat and angled refers to the shape or cut of the bristles. Flat bristles are great for wide areas or laying off paint. Angled brushes work great for having more control cutting in.
Block brushes are thick with flat cut bristles for brushing large areas and trim brushes are much thinner for fine control while painting trim.
It is up to you which will work better in any given situation.
The term sash is used indicate a flat brush. In my opinion, it is a general confusing term. I prefer to use the other terms for style; flat, angled, block or wall and trim.
Most interior painting on residential work can be accomplished using a flat edge, long handle, 2 inch Brush (commonly referred as a "sash tool") and a flat edge, long handle, 2.5 inch Brush for all the rest of the work…
Polyester Brushes work well for both acrylic and oil paint… Using angle edge brushes is for amateurs who have yet to learn good brushing techniques…