The stacking module is optional on the Firepower 8140, 8250, and 8350; and is provided in the Firepower 8260, 8270, 8290 and the Firepower and AMP 8360, 8370, 8390 stacked configurations. A Firepower Management Center creates, edits, and manages the stacked configuration. The primary device contains sensing interfaces and one set of stacking interfaces for each secondary device connected to it.