In this property you specify the device name of the PLC subdevice. PLC devices exist that consist of several integrated modules, meaning subdevices, and that have several start addresses. Such a device can consist, for example, of an internal CPU module, an internal input-output module as well as internal counter module - however with only one part number. In order to map such logical function units within a PLC device, up to twelve PLC subdevices are available to you.

Reference

Placement3DPropertyList Class
Placement3DPropertyList Members