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It is defined as the relation between the substituent and the structure with which they are stereochemically attached like homotopic, enantiotopic, and diastereotopic.

It is defined as the group in which 1 atom like hydrogen is replaced by another atom when the rest are fixed and the molecule remains the same stereochemically and produces the same chemical shift like compound a as when we replace any of the hydrogen represented by an arrow the stereo remains the same so homotopic.

Compound a

It is defined as the group in which 1 atom like hydrogen is replaced by another atom when the rest are fixed and the molecule changes its stereo and from mirror images and produces the same chemical shift majority by if have chiral carbon produce different chemical shift like compound b as the hydrogen represented by the arrow is replaced by another atom then stereo changed and mirror images are formed as shown:

Compound b

It is defined as the group in which 1 atom like hydrogen is replaced by another atom when the rest are fixed and the molecule changes its stereo but are not mirror images and produce the different chemical shift for every atom-like compound c as the hydrogen represented by the arrow is replaced by another atom the stereo of the molecule is changed as well as they do not mirror images as shown:

Compound c