Parties Involved on Objects Page

I am trying to find the Parties Involved assignment block on the WebClient UI Objects page (Component PRDHS). It does not show up under Available Assignment Blocks on Personalization, and I can't seem to find it in the BSP WD Workbench.

Does Parties Involved actually exist for the Objects page? If so, how do I set it as an Available Assignment Block?

Thanks for your help!