Project variables are unique to Impact and give Parametric designs a whole new dimension to work with. If a Variable is designated as a Project Variable it is instantly given the properties to be used in multiple layers across the project. In other words, you can assign this variable to different dimensions in different layers for the purpose of changing the value once and then applying it across an entire project.
To access this functionality simply click the Project Variables button to activate. All variables entered will be assigned these properties.
This type of variable can also be used in Expressions created for specific pieces of geometry in one or more layers.
This tool should be used when consistency in measurement is required for separate parts of multi Layer designs.