Offset of 16.7 Deg C or 30Deg F in TTRXB

Dear Sir
In my PG 5371PA, Mark VIe, a offset in TTRXB is given by 30Deg F, or 16.7DegC. that is (536.7)TTRXB= TTRX+30F.
During an increase in load (Isochronous Speed control, as it is an isolated system), if load increases to 18 MW, TTRXB comes down to 530 Deg C, from 536.7 DegC. IGV opening is 77 degrees. If IGV is controlled by TTXM which should be held close to TTRX then why TTRXB is needed and how it controls .
Sir please extend helping hand.

Ashutosh Mehndiratta