Center Distribution
The Center Distribution control is used to distribute part or all of the center signal to the left and right front speakers.
For example, this can be useful in the following situation: The center signal is panned directly to the center speaker and the Center Distribution is set to 0 %. However, the signal is too discrete for your liking, and you want to add part of the signal to the left and right front speakers to widen it. You can do this by raising the Center Distribution value. At 100 %, the center source is provided entirely by the phantom image created by the left and right speakers and using a value in between you can distribute the signal to the 3 speakers.
A blue line at the top of the surround field indicates the distance up to which a phantom signal is added. If you position the source signal inside this range, the signal is sent to all 3 channels.
Note that for this to work, the front speaker configuration needs to be symmetrical and there can never be more than 3 speakers involved.