Positional Invariance is a property of Relation: Inclusion and Exclusion stay distinct and ordered no matter how deep Reflexive Sequence goes.
Reflexive Sequence allows relational structures to deepen indefinitely — Relating relating to itself, again and again. This raises a question: does the basic distinction between Inclusion and Exclusion survive that deepening? If it did not — if, at some depth, the inside and outside could swap or dissolve — then Relation would not hold at that depth, contradicting what Relation is.
Positional Invariance is the guarantee that this does not happen. It is not an additional principle imposed from outside; it follows from the nature of Relation itself. For Relation to be the coherent maintenance of Inclusion and Exclusion, that coherence must be maintained at every depth.