What are virtual pins, and how to access signal at a lower hierarchical level

I have 2 questions. What is the point of virtual pins and how are they used?

Secondly I wish to know the method on how can one access the signals that are within a block e.g I create a circuit schematic and change it into a block, than I use a few of these blocks to create a new schematic and change that into a block. Now how can I access the lowest level signal wire between two logic gates in this case when I am doing 1) Pin assignment and 2) Simulation?