ThresholdClosure names the decisive operation through which Socioplastics [2510] converts accumulation into architecture: not by terminating movement, but by producing a seal that stabilises without ending. Situated within Core IV · Field Conditions, it functions as a threshold mechanism whereby a layer, tome, core, book, or conceptual stratum attains enough density, recurrence, metadata, citation, and navigability to become a durable unit while remaining available for subsequent activation. Its importance lies in its resistance to two opposing failures: dispersion, where everything remains open but nothing becomes legible; and petrification, where closure hardens into monumentality. ThresholdClosure instead defines closure as calibrated permeability, a boundary that intensifies returnability, consolidates relations, and generates stable reference without arresting metabolic circulation. In practical terms, this operation appears through DOI anchors, indexes, recurrent CamelTags, vertical spines, machine-readable datasets, public syntax, and persistent citation routes, each acting as infrastructural seams that allow the corpus to be entered, cited, taught, extended, and repaired. Its case is visible at scalar crossings such as 1K, 2.5K, 4K, and 5K nodes, where the field does not simply grow numerically but becomes newly readable as a structured layer. In dialogue with GravitationalCorpus, PortHypothesis, MeshEngine, ActivationNode, and AutonomousFormation, ThresholdClosure gives mass a contour, entry a position, and recurrence a citable form. It therefore anticipates later operators such as UnstableInstallation and HomoEpistemologicus, proving that in a living epistemic environment, genuine closure is not an ending but the condition that allows growth to remain coherent, public, and sovereign.