We've actually stopped using the term ML version in favor of Pega 7 Updates. But that doesn't mean you won't come across older documentation referring to the term ML, which I guess might be what prompted your post. So, when and if you do, ML just referred to the minor version number at the time the document was written (i.e. ML5 = 7.1.5, ML6 = 7.1.6).
Here's a good document explaining our Pega Update philosophy moving forward: