Hi Trxfred.
Curviness, as it relates to choosing a mattress, refers to the “gap” that needs to be filled in during sleep with the comfort layer for pressure relief. The “curviness” of body profile during sleeping is mostly assessed in relation to the sleeper’s primary sleeping position and also their body type.
Regardless of a sleeper’s body type, generally, back sleepers have a slightly less curvy profile than side sleepers, stomach sleepers have the flattest sleeping profile of all positions with fewer gaps to fill in. Combination sleeping is also more difficult to deal with than someone who sleeps in one position since the different positions have different basic requirements. You can read a bit more about this in this article about sleeping positions)
When considering the support or comfort a sleeper needs it is important to take into account a person’s BMI and where they “carry” their weight as heavier parts of the body will tend to sink in more and can cause alignment issues. Lighter weights will sleep in “flatter” sleeping positions, have slimmer less curvy body types, and might benefit from a firmer mattress. The hip/pelvic area carries the greatest amount of weight and has the greatest density, no matter what type of body you have, and often has a wider profile (especially in women).
A good example of a more unusual sleeping profile can be found in this post here which has some accompanying images that might be helpful for visualization. I’d recommend reading the entire thread which will show how different sleeping stats and variables are dependant upon each other.
Phoenix