The Peak section compares the inputs at PEAK 1 - PEAK 4 against each other and the MANUAL PEAK control, and outputs the highest voltage.

In a similar vain, the TROUGH section outputs the lowest voltage of the inputs TROUGH 1 - TROUGH 4 and the MANUAL TROUGH control.

Both sections can be used to combine waveforms, CVs or envelopes in unexpected ways while the PEAK section is often used for combining gates from a sequencer.

Both the Peak and Trough operate in the range of 0V to 6V, though in practice, you may find the Trough section does respond to input voltages going a volt or two below 0V.

http://www.elby-designs.com/contents/en-us/p947_ES84_-_Peak___Trough_Detector.html