@ChubbyQuark i'm sorry, i was confused bc i'm on mobile phone and i didn't check.
IfcCovering class has got only width, netarea and grossarea.
The sentence i wrote above is for IfcSlab that has width, length and depth.
Now i rewrite the sentence so people won't get confused.
So, regarding your last question, following the schema, an IfcCovering element can't have a length or depth quantity defined.
Of course, you can add another Property Set with these quantities, but it wouldn't be standardized. In this case, also, it's not possible to use the qto calculating function in object panel, but maybe the qto panel in n-panel can be helpful.