OK it is very simple.
Talent is in the booth, Client is in the control room. We are doing a simple VO to DAT.
I need my slates to go to tape. which the talkback button achives. BUT . . . I, and my client also need to have a regular conversation with the talent in the booth in order to direct them. So, yes the talkback mic stays open while talent is directed.
Hope that clarifies the objective