For the given design, every row stands for a sample point (level for the system) that is a series of factor settings.
Minus sign (-) denotes low level setting, plus sign (+) denotes high level setting. For each factor, levels are decreasing from +3, +2, +1, 0, -1, -2, -3 and so on.
The 12x20 matrix below is our design for this problem.
In this case, for the first design point, we use high level setting for Factor 1 through Factor 13 and low level setting for Factor 14 through Factor 20.