The returned collection contains all parameters defined for the test or component and their default values according to the order in which they are defined (all input parameters, followed by all output parameters). 

You can check whether each parameter in the collection is an input or output parameter using the ParameterDefinition.InOut property.

You can use the Parameters object and Parameter object properties to modify the values of the returned parameters before you supply the returned object as an argument to the Test.Run or Test.UpdateRun method.

Retrieve the Parameters Defined for a Test

ParameterDefinitions Collection