In the movie,while the Corleones are waiting to send Michael to the Sol hit,there is a mention of the "negotiator" playing pinochle with Clems men,amd that until Mike comes back safe and sound,he keeps playing cards.
I wonder if Coppola used that phrasing instead of 'hostage' to avoid going into an explanation of the Bo's. I think by this time in the production, only stuff really critical to the story was going to be included.