WARNING The following is an extreme over simplification....
Newer wood carvers soon discover what is labeled "flat plane carving". It's always easier to show examples of flat plane carving than try to explain with mere words what flat plane carving is. It becomes even easier to make the distinction between traditional caring style and flat plane style with examples.
It's even easier to show the differences when you use exaggerations like I have done.
The example on the left is traditional carving, and on the right is flat plane carving.
On the left, traditional. The right flat plane.