Th M-Check for fish assesses how responsible or "sustainable" the product is. The number of stars is thus an indicator of how responsible a product is – the more stars a product has, the more responsible its production.
"Responsibility" is the variable that is measured, as sustainability can be defined in a variety of ways and a testimony regarding the "sustainability" of a product can be ambivalent in practice.
In the M-Check rating system, transparency is a priority – the aim of the star rating is to make it possible to compare products with one another.