Its the same case in report definition and data page as well. A parameter is a value passed into a rule to make it more reusable. Parameters enable you to separate the functional behavior of a rule from specific business contexts, and therefore, maximize the reuse of the rule.
Use this parameter field to indicate whether the parameter is used for input to the rule, or returned as output from the rule.
The In/Out field is required when displayed. The In option is selected by default.
I have found one pega academy topic as well which is giving the whole description with the examples, named as "Parametrizing rules for reuse" part of senior system architect course., please go through it.