It is a profile you create or assemble if you will using existing moulding profiles, import, or create yourself.
You can do the window stools and apron as one profile, or as individual components.
If you make a profile of both combined & create a symbol, save them in a library and name them to your liking for future use.
Tips that may help you are;
How many different casing profiles (moulding styles/sizes) do you use? 2-1/2", 3-1/2", casing, etc...
How many different stool profiles do you use?
What depth stools (wall thickness driven) typically.