SK-AM62B-P1: looking for further explanation about 「private key bus」

Part Number: SK-AM62B-P1

Tool/software:

Hi TI,

We are planning to enable the security feature on SK-AM62B-P1 and I am reading the following reference to understand the security architecture of TI devices.

Securing Arm-Based Application Processors (Rev. E)

Would you please provide further explanation about「private key bus」which is underlined as follows?

I investigated related TRM but couldn't figure out what is this「private key bus」.

Best regards,

Xie