PGA400Q1EVM: PGA400-Q1 TEST register 0xFF

Hello,

The PGA400-Q1 EVM User's Guide Rev. A has following description (sldu010a, page 21) ;

"6.2.2 Restricted Access
As described in the PGA400-Q1 data sheet (SLDS186), in order to access some of the features in the
TEST register map, a special unlock sequence must be sent to register 0xFF. The Restricted Access
button sends this sequence to the PGA400-Q1 device."

But I cannot find the description of the TEST register-0xFF in the user's guide and the data sheet.

What are the register-0xFF and the "special unlock sequence"?
Is there a document which explains that?

Thank you in advance.

Takishin