Anything on the table is open to reference by anything else on the table afaik. Direct references are usually done as cursors projected off the page so you can "point" and "click" on other objects to control precisely what you reference or select with the other tool.

You could also implement input and output piping between programs in a more organized way where their physical orientation isn't as critical as most page references seem to be. eg: Put a tag on a sticky note that represents a pipe, put it close enough to the 'output' of one page then stick it to the input of another compatible page.

Not sure how protected the individual pages are from outside modification because real details are quite thin on the ground. I think right now you could probably turn every display page into a rick roll if you wanted.