Max VG refers to an e-liquid that contains vegetable glycerin VG at a maximum level, with minimal or no added propylene glycol (PG). Because of vegetable glycerin’s organic properties, max VG e-liquid will be thicker than other e-liquid, increasing the time it takes to soak into the coil.
Max VG doesn’t refer to a specfic ratio of vegetable glycerin (VG) and propylene glycol in e-liquid
Max VG are generally recommended for sub-ohm tanks that pushes 50w or more because they tend to have a larger coils and with more cotton which makes it more susceptible to leaking. Max VG are tend to use with RDA or RDTA because it allows a thicker cloud production.
Due to the thick nature of Max VG, standard tanks that runs 20w or below would have trouble soaking/wicking into the coil and would result in dry burning hits, which would lead to the coil burning out faster.
AFAIK Max VG refers to juice with no added PG - seeing how flavouring concentrates are mainly based in PG, its unusual to see all-PG, however sometimes people are looking for the most VG possible (usually they claim to be a part of the 0.1% of people with a PG sensitivity). The only PG in Max VG juice would be that already in the flavouring concentrates, which is why it is often negligible - in comparison to PG and VG, the flavouring concentrates would be minor; even measured in drops.
A juice can be 80% VG and not Max VG - what makes it Max VG is that it has the maximum amount of VG possible, or no added PG.