In my opinion, it propably could be treated only like a kind of customary law.
But for example the FSC standard of responsible forest management consists of 10 Principles and 56 Criteria set at the international level.
According to Rule no 1 FSC party countries comply with all binding international agreements, in particular CITES, ILO Conventions, ITTA and the Convention on Biological Diversity.
FSC rule no 4 states that the right of employees to associate and consult with employers is guaranteed by the provisions of International Labor Organization conventions 87 and 98.
FSC recognize also many other international regulations and EU regulations as well national legal orders.
Even if we do not recognize that Voluntary Sustainability Standards (VSS) are legally binding, it is impossible not to notice that in themselves they refer to important national and international regulations. So, indirectly VSS serve the important function of regulating and systematize these two legal orders. In this way, they allow to better understand the essence of voluntary adoption of certain standards or to undergo independent certification processes. It seems to me that the gradually growing consumer and civic awareness favors not only these processes, but also the conscious decision making by individuals in various aspects of social life.
Read Scott's and DiMaggio's sociological work on institutions. Regulative institutions are what you consider legally binding, whereas normative ones are those that form expectations in behavior, so if everyone in my industry does voluntary disclosure and I don't - even if its not illegal, I have strong forces influencing my decision to disclose.