Complex sentence: Some composers choose just to subject the pitches of a musical line to retrograde, or just the rhythms, and in twelve-tone technique reversal of the pitch classes alone, regardless of the melodic contour created by their registral placement, is regarded as a retrograde.
Split into simpler sentences: Some composers choose to subject just the pitches of a musical line to retrograde, or just the rhythms. In twelve-tone music, reversal of the pitch classes alone--regardless of the melodic contour created by their registral placement--is regarded as a retrograde.