Codebook subset restriction IE under RRC connection reconfiguration

Specs & Protocol Testing
1

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

2

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.

3

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