The PDF/VT specification was published by the International Standards Organization in August 2010, following 3 years of collaboration by industry stakeholders around the globe. It is part of a family of PDF subsets optimized for a specific purpose (e.g. PDF/X, PDF/A, etc.). The "V" stands for Variable, and the "T" stands for "Transactional". The standard is formally known as "ISO 16612-2 – Graphic technology -- Variable data exchange -- Part 2: Using PDF/X-4 and PDF/X-5 (PDF/VT-1 and PDF/VT-2)". The specification can be purchased from the ISO at http://www.iso.org/iso/catalogue_detail.htm?csnumber=46428.