Hi I need to segment each lung volume into 6 segments using well-defined landmarks but I do not know how to create these additional segments

Support
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Operating system:
Slicer version:
Expected behavior:
Actual behavior:

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Did you look at the Segment Editor documentation? Image Segmentation — 3D Slicer documentation

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Which well-defined landmarks do you have?

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I would start with using these 2 planes:
-one horizontal plane touching the inferior aspect of the end of main bronchi(i.e., when they divide) to dissect upper from lower lung segments
-one coronal plane passing trough the anterior aspect of the vertebra T12