What Does It Mean for a Variable in the Application

What does it mean for a variable in the Application Editor to be defined as a parameter?

A) The variable can be used to pass data to and from subflows.
B) The value for that variable can be supplied via Application Configuration in Application Administration.
C) The value for that variable is defined by the calling application.
D) The variable can be used in conditional steps.
E) The variable can be used to pass data to and from VoiceXML applications.