Difference between Hidden and Internal in Parameter visibility ? in SSRS

let me say you Develop a Report which has a parameter as  User!userID.Value (Named as EmployeeID in Reports)  to Filter your Data in the Reports and this is captured when user logs into Report Manager.

Case 1:

If you have this EmployeeID  as Internal you cant change this Value by passing this Value in URL.

Case 2:

If you have this EmployeeID  as Hidden and Deployed it..you can OverRide User!userID.Value by passing this Value in URL Like this ..

http://server/reportserver?/Sales/Northwest/Employee Sales Report&rs:Command=Render&EmployeeID=1234

Employee 1234 can pass 1235 and see his Data which will be security threat.

Hope this Clarifies your Doubt.