Modules

Modules 1:
Expand Solution-Explorer > TwinCAT Project > SYSTEM > TcCOM Objects > Object (XtsProcessingUnit)
Right-click on Object (XtsPart) to open the context menu
Click Add New Item... in the context menu
Modules 2:

The Insert TcCOM Object dialog box opens.

Expand Beckhoff Automation GmbH > XTS Modules
Select AT2xxx-0xxx [Module]
Enter the number of modules required in the input field Multiple
Double click on the selection or confirm with OK

One or more modules are added to Object (XtsPart).