Hey Tim, thanks for clarifying!
Yes -- those stories from other projects can be linked via the Dependencies tab in Rally. Really, it's depending on how you have your project scoping set via the Project Picker, as anything outside of that scoping wouldn't be able to be seen from certain projects.
As an example, please consider the following:
If I have my hierarchy set up like:
- Project A
- Project B
- Project C
If I'm in Project C, and even if I have Show All Items from Parent / Child selected,I will not be able to see items from Project 2
as it is outside of the project hierarchy I am currently in. I would only be able to see items from MAIN and Project C. So, you can see that this would affect which Stories you will be able to add as Dependencies for the selected story.
Does that make sense?
Here's a little more on Project Scoping: https://help.rallydev.com/change-project-scoping