Slate will show the cells that are in another document but referenced by the current sheet. However, it would not show any further levels of referenced cells.

So say I reference the value "Total Sales" which is in another sheet. That would show in the the tree but the individual sales that contribute to the total would not be shown.

We wanted people to know where the numbers are coming from but at the same time keep the trees for different sheets and files separate to prevent them becoming too unwieldy and hard to understand.