They both mean the same.
When you 'Link' Main Report prompt with a Subreport prompt/database field, you're still essentially 'passing' the parameters from the Main Report to the Subreport.
You can also 'pass' values from the Main Report to the Subreport (using the same 'Change Subreport Links option) just for the purpose of using that value somewhere in Subreport's formulas.
-Abhilash