Codebook subset restriction IE under RRC connection reconfiguration

Hi for LTE Experts, one question.
There is IE codebook subset restriction under RRC connection reconfiguration.
What is significance of this IE?

I remember in Samsung they use virtual matrix (VM) to address the power imbalance problem among the ports for 4CRS TDD 20 MHz.
For this purpose, some codebook index were restricted.

It’s part of antenna info dedicated IE related to codebook.