Formal Notes on the c²-Model VI – Protocol-Relative Accessibility and Structural Distinguishability

Formal Notes VI develops an operational refinement of observational-informational accessibility within Version 9.0 of the c²-Model of Realities. The study distinguishes physically realized information acquisition, task-relative structural distinguishability, observational underdetermination, model-dependent reconstruction, and reliability. A physically admissible measurement protocol associates structural descriptions with probability distributions over possible observational outcomes. Structural distinguishability is defined relative to specified scientific tasks, measurement protocols, and discrimination criteria. Multi-operator observations may reduce observational equivalence classes without guaranteeing complete structural identification. In the linear case, residual structural blindness is characterized by the joint null space of the observational operators. Coarse-graining and data compression are examined with respect to information preservation and task-relative sufficiency. The framework further distinguishes direct observational evidence from model- and prior-dependent reconstruction. Reliability is assessed under specified observational uncertainties, including correlated errors, unknown calibration parameters, and possible systematic distortions. Three scientific examples illustrate task-dependent measurement quality, complementary observational channels, and independent testing of reconstructed physical properties. Formal Notes VI refines the observational logic of the c²-Model without introducing new ontological assumptions or claiming empirical confirmation of the proposed Basis Reality (BR) to Sub-Reality (SR) projection. The resulting framework provides a methodological foundation for the future formulation and evaluation of empirically distinguishable consequences of the c²-Model of Realities.