There are many accessories for use with studio flash and even for external flash. Some will produce a hard light and other stray light.
Then we use the fitting will determine the quality of light.
If an accessory use these flash light will be scattered in all directions, will reflect on any surface and will be very difficult to control the lighting.
When we use directional light fixtures to illuminate what we want.
Before seeing the accessories we will see that means a hard light and diffused light.
Hard light: the light arriving directly to the subject or object from a single light that is not reflected by any surface.
Hard light produces images with high contrast: dark shadows and bright lights or white.
The shadows have a well-defined edge. If we seek an example of hard light this could be the sunlight. The light comes from a single direction and light source and creates dark shadows.
Diffused light: scattered light is generated when the light comes from a very large light source (usually near the object) from many directions, or multiple light sources.
This type of lighting produces images of lower contrast and transition between light and shadow is not so hard.
An example would be the light on a cloudy day. The light passes through the clouds and becomes diffuse, and as the light comes from many direction, the whole sky light actually becomes soft.
This photo was taken with a studio flash but using a fixture that generates scattered light (umbrella).
You can see that the picture has less contrast than the previous one, the shadows are not so dark. The transition from light to shadow is much milder, in fact, is a kind of degradation.