What is the meaning of the input for "Percentage of Edge Length for M&V"?

It is a way for the user to apply engineering judgment in the determination of the soil pressure to use for design.

Think about a footing with a linear-varying soil bearing pressure where the pressure is high on the east and low on the west. If you consider the design of the bars that run North-South, you would need heavier reinforcing for the N-S bars that are on the east, and lighter reinforcing for the N-S bars that are on the west. But structural engineers generally want a consistent size and spacing of rebar in a particular direction in a general footing. So the question boils down to deciding what pressure to use to design those N-S bars. If you used the lowest soil pressure, the rebar could be under-designed for a large portion of the footing. If you used the highest soil pressure, the rebar could be over-designed for a large portion of the footing. Most engineers are going to select some value of soil bearing pressure that is somewhere between the lowest and the highest soil bearing pressures experienced along an edge. That’s what this control is really doing. It is specifying the percentage of the footing dimension over which to average the soil bearing pressure to arrive at a single value that is judged to be appropriate for design.

If you use 100%, then you are telling the program to use a value that represents the average pressure along the full length of the footing edge.

If you use 10%, then you are telling the program to use a value that represents the average pressure along the most heavily loaded 10% of the footing edge.