Boris Borisov
I think I included this pic before. For the smaller circle, as discussed this could be additional seating; a feature plant or other type of feature. If it is to be planting, then I would suggest that at ground level we use a contrasting material such as gravel and if it is seating, it could be gravel; paving or timber, set within the bed and grass as on the drawing.
Planting in varied heights
Q