In Sewnet, we have different Manning coefficient for the new and old pipe. When pipe ages, the Manning coefficient will change, because the roughness will be changed. This is why we have two sets of parameter for Manning coefficient
We included both of the new and old pipe Manning coefficients because the authority may require that you submit both.
If you only want one calculation, you can set the Manning coefficient for new and old to be the same in Parameter->Material->Pipe Type