Go to Tools > Flatten Status 
The colors on the electrical geometry change to indicate you whether the different branches are flattenable or not.
- Green: the branch is flattenable.
- Yellow: the branch is not flattenable.
- Blue: the branch is already flattened.
Click the branch you do not want to be flattened.
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The selected branch turns yellow. Click again if you want it to be set back to green.
Double-click the background to validate and exit the command.
Use the Flatten
command.
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Use the Flatten Status command again to see the different branch statuses.