A common operation within logit models is to combine two or more sets of o-d cost matrices at a common “level” in a hierarchical demand model into a single composite perceived cost matrix via a “log sum”. See, for example, equation (7.27) in section 7.6.3. In principle a log sum of two or more matrices may be achieved by using the Fortran Equation options above although the equations tend to be rather long-winded and easy to mistype; hence the need for an explicit option.
To carry out a log sum the user must (a) identify those matrices to be combined (presumably cost matrices but no explicit check is possible) and (b) a value for the parameter beta. The composite matrix is created as the internal matrix.