Illustrated Glossary of Organic Chemistry

Diastereotopic: Atoms or groups that when replaced lead to diastereomers.

or
(R)-2-bromobutane
(2R,3S)-2-bromo-3-chlorobutane
(2R,3R)-2-bromo-3-chlorobutane
The methylene hydrogen atoms of (R)-2-bromobutane are diastereotopic because when each is replaced by a chlorine atom, (2R,3S)-2-bromo-3-chlorobutane and (2R,3R)-2-bromo-3-chlorobutane (a pair of diastereomers) result.