In capella 7 and capella 8 there is a feature we call "living voice extraction". During living voice extraction the score remains unchanged, it ist just partially hidden.
That is, when you select the command Extras - Voice extraction: Selection... you only see the selected voice(s).
Sometimes desperate messages reach us: "My score is gone!" No, it's not.
Imagine looking at the score through a small observation slit - you will see only the selected voice. Everything you edit in this voice will be automatically edited in the entire score as well, because you are still located in the score (you just don't see everything).
In capella 8, pay attention to the yellow display bottom right and the yellow button top right. With this button you can at will switch between voice and score.
When you have reached the last stage of score editing and are now optimizing system breaks in the voices and joining rests, you need to separate voice extraction from score. To do so use the command File - Save voice extraction... It is important that you understand that from now on the voice extraction will be a separate file which is not connected to the score any more. It has a new name, and the yellow buttons (see above) are gone.