Add Subsystem

Setup → Manage Assemblies → Add Subsystem      Shared Dialog Box
 
For the option:
Do the following:
Assembly Name
Select the assembly you wish to modify. The menu lists all assemblies open in the session.
Subsystem(s)
Select one or more subsystems:
■To search/browse for a subsystem, right-click the text box next to .
■To select a subsystem that is open in the current session, right-click in the field next to and select a subsystem.
Variant Options
Select one of the following options:
■Update existing variant
■Create new variant (and supply a new variant name)
■Don't update variants
Shift Subsystems
Opens the Subsystem Shift dialog where you can enter a longitudinal and vertical shift for each subsystem specified.
Subsystem Variants
Opens the Subsystem Variants dialog where you can select the subsystem variant for each subsystem specified.