The step is enabled in the case of a gang job with the "Gang & Sheet Optimizer" option only (see also Ganging Optimization).
When you have a gang job, you can set various parameters for optimal distribution of the 1ups on the press sheets in the "Optimization" step.
These parameter values are then used to calculate the position of the 1ups/articles and the number of required press sheets taking into account the geometry data and the quantity (part of the xml or csv file).
Optimization is based on geometric variables like sheet margins and paper size and also includes cost-related parameters.