Mattress support shouldn’t be confused with mattress firmness. The support corresponds to the sagging of the mattress, its resistance to sinking, but also to its stability and cushioning. A good support for the spine will prevent back pain, for example.
Firmness is primarily about the feeling of comfort when you lie down on the mattress. It depends on many factors such as the fabric or the thickness of the filling. There are various levels of firmness: very soft, flexible, balanced, firm, very firm.
Regardless of the desired level of firmness, don’t rely on what the label says. There’s no standardised scale of firmness, so each manufacturer defines it in their own way. Note that many manufacturers provide tables that relate mattress shape to firmness. However, there’s no hard and fast rule! It’s up to each person to choose according to their preferences. The important thing is to choose a mattress that supports your body, especially your spine.