What does the constraint in the parameter for a dataset actually do?

Hi, I'm trying to get the constraint for a parameter in a dataset to work, with no success, so I just wonder what exactly does it do, and how can we constraint the objects being displaid in the parameter that is used in a simple chart (drop down). relates to this question also
1 answers

I have noticed that in order for the constraint to work, you should update the security of the module (after creating the parameter's constraint) and apply the constraint for the user viewing the report because by default the user has full access on the data set.