For me there is an almost instinctive choosing of
what edges I make soft and which ones are painted
with a crisp edge, a sharp demarkation.
Here you see the edges of the arm are soft and the
edges of the fabrics are mostly crisp and sharp. Flesh
is soft and so soft edges make sense in order to convey
this quality. The fabric elements seen here, are rather
flat and are one layer over another, so sharpness of each
edge helps convey that.
Often I will paint an edge 'hard' and then I will use a sable
brush to blend it and soften it up almost imperceptibly.
Sable brushes are mainly for such blending and softening
because the bristles are not stiff and will not move the paint
around much ~ just enough to blend things.