Determining contact pair surface on inside of a block

[bcj17]

I have a vasculature branch that I want to embed into a block. I cut out the volume of the vasculature inside the block, but am having trouble with the contact pair. How can I define the inside surface of the block? I easily defined the surface of the branch but dont know how to define the same surface on the inside of the block. If I were to use the entire surface of the block would that work also?

 

[Mustaine3]

A/CAE allows selection on the outside of a structure by default. If you want to select something at the inside, you have to change the options in the selection filter.

See CAE Users Guide 6.3 Using the selection options