about superimposition

Operating system: 3d slicer (window 64bit?) I don’t know what exactly mean…
Slicer version:4.11
Expected behavior: 3d model superimposition
Actual behavior: hello, I’m user of 3d slicer 4.11 version. Then I have a question of function.
When I use superimosition module through volumn rendering tap, (I usually superimpose pretreatment and possttreatment volume with CT bone preset) I want to control opacity of posttreatment volume. but I don’s know the tap of it. Is there any module which I must download?
Thankyou for your helpp

Would you like to display pre and post treatment volumes in 3D, in the same space, using volume rendering?