What is the diffusion circle in photography? What's the effect on taking pictures?

Before and after the focus, the light began to gather and spread, and the image of the point became blurred, forming an enlarged circle, which is the diffusion circle. In reality, if the diameter of the diffusion circle is less than the resolution of human eyes, the blur produced by the actual image is unrecognizable in a certain range, and this unrecognizable diffusion circle is called the allowable diffusion circle. The distance from the focus to the near allowable diffusion circle is called the foreground depth, and the distance from the focus to the far allowable diffusion circle is called the back depth of field. In fact, it is the existence of diffusion circle that forms the depth of field, and the diameter of allowable diffusion circle is also an element that affects the depth of field. Different camera frame sizes and photo magnifications lead to different allowable diffusion circle diameters.