Never heard of an "I".
Proper subsets of PDF have been, or are being, standardized under ISO for several constituencies:
PDF/X for the printing and graphic arts as ISO 15930 (working in ISO TC130)
PDF/A for archiving in corporate/government/library/etc environments as ISO 19005 (work done in ISO TC171)
PDF/E for exchange of engineering drawings (work done in ISO TC171)
PDF/UA for universally accessible PDF files
A PDF/H variant (PDF for Healthcare) is being developed. However, it may consist more of a set of "best practices" than of a specific format or subset.