UCC28950: Question about COMP pin

Part Number:UCC28950

Hi team,

   I want to know in which condition UCC28950 will have maximum duty output?

• When V(comp)>2.8?
• When V(comp)>2.8+0.85?

    And I would appreciate it if you can share me the reason why we have 0.85V offset voltage in below picture. Thanks a lot!