How do DDI InstanceVariables link or correspond to SDTH VariableInstance?

[DanSmith]

[altergc]

An SDTH:VariableInstance refers to a set of values including their order, coding, formatting, and other attributes. This means that a VariableInstance can exist in more than one DataframeInstance and/or FileInstance. It also implies that a VariableInstance is an abstraction, because it is not tied to a single instantiation.

How is DDI Instance Variable defined? Is a DDI Instance Variable is defined by its existence in a physical data file at rest?