Access common (part program) variable using API

Access common (part program) variable using API

Hardware: Okuma OSP200L
API Version: 1.15.0.0
How can I read and write the values of common variables?
I found the example code in the help file, but it doesn't explain how to
set the current sub system. Also I am confused about the difference
between AddCommonVariableValue() and SetCommonVariableValue() Could
someone please give a clear example / explanation?
===GET===
Me.varValue.Text =
bjVariables.GetCommonVariableValue(CInt(Me.varCommonVarNumber.Text))
===SET===
objVariables.SetCommonVariableValue(CInt(Me.varCommonVarNumber.Text),
Dbl(Me.varValueUpdate.Text))