It's a 4 channel amplifier that allows one to combine channels to distribute power accordingly.
According to the manual (you read those, right?
"CXD amplifiers feature Flexible Amplifier Summing Technology (FAST). Depending on the model, 1600, 2500, or 5000 Watts of total power can be distributed across one to four outputs. In addition, the CXD4.3 and CXD4.5, can drive 70V or 100V speaker lines directly from any one or all of the four outputs. This flexibility allows CXD Series amplifiers to drive (for example) two full-range, surface-mounted loudspeakers along with a subwoofer and one 100 V distributed loudspeaker line; or a high-power subwoofer and a bi-amplified full-range loudspeaker; three 70V distributed loudspeaker lines and a subwoofer; or a single very high-power channel to drive monster subwoofers".
Here's the white paper on the technology that enables this:http://qsc.com/files/7813/7088/4198/FAST_White_Paper_06062013.pdf
A very versatile power amplifier!