A questions came up on a support call the other day, asking how to insert a head office opex module into the Multi-Business Unit content.
As background to the issue: when inserting an opex module into a multi-BU model, the additional opex categories are added to the historial income statement, which also triggers the modules-as-categories mechanism, and adds three new business units.
The key to circumventing this is that you need to first insert spare categories into the historical income statement - via Insert rather than Insert Categories.