In some comments, there is "HasTangentArray" while others not.
I wonder what the "HasTangentArray" means .
Thank you!

What's the differences between the group with "HasTangentArray" and without it?

You need Tangent Normals for bumpmapping. The original normals are not saved on import, but they can be generated on export. Saving the originals would be useless if the mesh was modified very much.

The HasTangentArray: comment notes that the original had the bumpmap normals, and the exporter plugin will regenerate them. You can stop this by deleting that comment.

You could also add them to a mesg group by inserting that comment. For bumpmapping to work, the texture used also needs to support the bumpmapping.

<* Wes *>

Thank you very much , wes_h !!!
I understand now .
Good explanation !