Outside of the VR2 policy, where can I learn more about mass balance and credit systems?

Introducing mass balance is a new scope for Textile Exchange.  We will add resources as the concept progresses. In the meantime, other schemes have information based on long-standing processes. ISEAL chain of custody models is a good resource that covers many different sustainability certification systems. Schemes with mass balance options include Rainforest Alliance, Roundtable on Sustainable Palm Oil (RSPO), and Forest Stewardship Council (FSC).