What is called an image set in a database

the essence of an image set is one-time selection operation and one projection operation. 
For example, the relationship pattern R (x, y), X and Y represent two sets of attributes that are complementary to each other, and for a relationship a that follows pattern R ,
when T[x]=x, X's image set (Images set) in A is:
zx={t[z] | t∈a,t[x]=x}
It represents the projection of the set of tuples in a with X component equal to x on the attribute set Z. 
such as a:
X Y Z
A1 B1 C2
A2 B3 C7
A3 b4 C6
A1 B2 C3
A4 b6 c6
A2 B2 C3
A1 B2 C1
A1 The image set in A is {(B1,C2), (B2,C3), (B2,C1)}

What is called an image set in a database