How do you describe to someone the "firmness" of an object? Usually you would make a comparison with some other object, i.e., "as hard as a rock", "as soft and light as a feather". How about just the concept of "firm" itself? It's rather subjective. A lot of consumers have said over the years that "my mother told me to buy a firm mattress". What did she mean? Firm as in "hard", or firm as in "supportive"? There's firm/hard like cement, or there's firm/supportive like the cushions on a new sofa. You would have a hard time dozing off while lying on a cement floor, but you probably, quite often, doze off while lying on the sofa.
Envision your living room with wall-to-wall carpeting, a chair or two, a TV, a sofa. Your carpeting has a decent pad underneath it and, sometimes, you like to stretch out on the floor and watch the tube or read a paper. This is what would be considered very firm. Not much padding and the padding is, in itself, firm. This type of mattress construction will not be that comfortable to most people but, over time, it will also show the least amount of "body signature".
Now envision that the floor has been okay to lay on for awhile but you're feeling a bit of pain on your pressure points. So you take the cushions off the sofa and put them on the floor and lie down on that. Mattresses of this type construction use significantly more, yet firm, cushioning material over a supportive core. It's still described as "firm", but it's a lot more comfortable.
Using the same living room setting, you put the cushions back on the sofa and lie down. Now the support structure beneath cushions is a little more forgiving than the floor. Mattress constructions can accomplish this feel through the use of different support cores and/or using softer cushioning materials. Such constructions will be a bit more prone to developing "body signatures."
Say lying on the couch is still a bit too firm, in your opinion, so you bring out the comforter from the bedroom and put it on top of the sofa cushions. Mattress constructions of this nature often use different techniques like pillow-top styling to allow the cushioning to "float" and conform to the body. "Body signatures" will be more pronounced but, in order to get the plush softness, more and softer cushioning is used.
The sofa with the comforter on it is still too firm. So you go back into the bedrooms and put all your pillows on top to the comforter and lie down. Oooooh! Aaaaah! Soft and comfy. Mattresses of this type use quite a lot of soft layers over a supportive core. Although prone to body contours, the layers and layers of cushioning usually remain very comfortable over a long period of time. Some constructions that bring very, very good cushioning materials to the top surface may not show much of a body signature but still be very soft and conforming.