Upper structure voicings split the chord into two simple parts: the left hand plays the tritone (3rd + 7th of a dominant chord) while the right hand plays a major triad. The triad adds altered extensions — which ones depend on which triad you choose. It is the most elegant way to voice complex altered dominants.
LH: 3rd + 7th of the dominant chord (the tritone). RH: a major triad built on bII, bIII, IV, #IV, bVI, or VI above the root. Each triad produces different altered colours.
Over dominant 7th chords when you want altered colours (b9, #9, #11, b13). Essential for jazz ballads, turnarounds, and any time you resolve to a tonic. The beauty is cognitive simplification — you think 'triad over tritone' instead of remembering individual altered notes.