KERML_ — direction, semantics

Feature direction (Feature::direction:FeatureDirectionKind) is described as

in, out, inout: Specifies the direction of a feature, which determines what is allowed to change its values on instances of its domain:
-in: Things "outside" the instance. These features identify things input to an instance.
-out: The instance itself or things "inside" it. These features identify things output by an instance.
-inout: Both things "outside" and "inside" the instance. These features identify things that are both input to and output by an instance.

but this semantics is not math/modeled and depends on Kernel for instances changing over time.