The two structures shown (left and right) are stereoisomers of 2-bromo-3-chlorobutane, $\mathrm{CH}{}_3-\mathrm{CHBr}-\mathrm{CHCl}-\mathrm{CH}{}_3$. They are related as

The left structure is (2S,3R)-2-bromo-3-chlorobutane and the right one is (2R,3S). Both stereocentres are inverted, so the structures are mirror images. Unlike 2,3-dibromobutane, the two centres here carry different halogens, so no internal mirror plane is possible and no meso form exists. (2S,3R) is therefore chiral and (2R,3S) is its enantiomer.