What makes a requirement binding here
None of its own, deliberately, and the history explains why. PIC — the Pharmaceutical Inspection Convention — was founded in 1970 by EFTA as a legally binding treaty for mutual recognition of inspections. Legal bindingness turned out to limit who could join, so in 1995 it was supplemented by the Pharmaceutical Inspection Co-operation Scheme, an informal, non-binding co-operative arrangement between authorities. The two now operate together as PIC/S. It is not a treaty, it issues no law, and it has no jurisdiction over any company.
Only through a participating authority adopting the guide into its own framework, at a version that authority chooses. The PIC/S GMP Guide, PE 009, is the working document; membership belongs to regulatory authorities rather than to companies, and is open to an authority operating a comparable GMP inspection system. What PIC/S actually produces is confidence between inspectorates — training, common standards and voluntary exchange of inspection reports — not a permission a manufacturer can hold.