Bondarg

Common to the left side: Images represent the INTERSECTION of a solid square and a solid circle. The black shape shows only the overlapping area where both shapes exist simultaneously.

Common to the right side: Images represents the UNION of a solid square and a solid circle. The black shape shows the total combined area of both shapes added together.

Left: the union of a square and a circle forms a single shape; right: squares and circles are joined only by intersection, as they are individual shapes.

intersection between square and circle vs union

Less concrete: left hand side shows a main shape with parts chopped off of it, right side is a shape with parts added to it

So first I noticed that in both figures there are only parts from the circle or a square, secondly I noticed that the secon section doesnt have any shape cut off (ignoring the overlap since you cant draw this in 3D). Most fitting pattern is probably that the lefthand side is the only the overlaping part (A and B) of those figures and the second part is the entire space that those 2 figures reside in.

if i were to express it more clearly i would say
A - Square
B - Circle
lefthand side = A and B
Righthand side - A or B