Rotate copy... Do we need it?

For a perfect alignment we could use 'Copy Selection/Center Copy' but I don't understand the usefulness of the option 'Rotate Copy'.

I have made several tests with 'Rotate Copy' checked and unchecked and the results was the same... So 'Rotate Copy' do something?

With Rotate Copy turned off, the copy will take its rotational values from the source object (the currently selected object). With Rotate Copy turned on, the copy gets its rotational values from the target object (the object you click on to rez the copy). Of course, if the source and the target happen to be the same object, then Rotate Copy will never make any difference. I suspect that that's what you had been doing in your experiments, and that's why you didn't see any change with RC turned on or off. Try it with the source and the target as two different objects, and you'll see the difference.

To see what I mean, try this:

1. Create two cubes. Color one red and one blue to keep track of them easily

2. Rotate the blue cube 45 degrees on any axis.

3. Select the red cube, turn on Copy Selection, but do NOT turn on Rotate Copy. Click somewhere on the blue cube, and you'll see a copy of the red cube appear, touching the blue cube at wherever you happened to have clicked on it (or centered if you had Center Copy turned on too). But the red copy doesn't at this point match the blue cube's 45-degree rotation. It's still at zero on all axes, since that's what the original red cube was at. The red doesn't know or care about the blue cube's rotation at all.

4. Now turn on Rotate Copy and do the same thing. This time you'll see that the red copy comes into existence already rotated at 45 degrees, to match the rotation of the blue cube.

That's all there is to it. Rotate Copy is quite simple, and highly useful.

Thanks a lot Chosen for your explanation. Now (finally) I understand 'Rotate Copy!