What does the m_n_extra_dims mean?

[hijkzzz]

In include/neural-graphics-primitives/nerf_network.h:

uint32_t m_n_extra_dims; //extra dimensions are assumed to be part of a compound encoding with dir_dims

But I still can't understand what these extra dims refer to.
Also I can't understand the train_extra_dims in src/testbed_nerf.cu.

Thanks~