As with CTM, the SNP Optimizer can be run in net change mode. This allows the fixing that occurred in the previous optimization run to be maintained. This is well described in the quote below:
“Incremental optimization defines how to deal with results from previous planning runs. If the Do not delete any orders indicator is not set (see below) or if fixed orders exist the algorithm might run into infeasibilities if the existing orders are considered as hard constraints that need to be fulfilled. To influence this, demand originating from fixed (and existing) orders can be treated either as a hard constraint, a pseudo-hard constraint…a corrected forecast, or a forecast. – Real Optimization with SAP APO
However, in most cases this indicator is disabled and the optimizer recreates all orders during every run. This of course takes longer, than if this not performed. The authors do a good job of explaining the fixed orders themselves as constraints.
“Real Optimization with SAP APO,” Josef Kallrath, Thomas I. Maindl, Springer Press, 2006