Hi Guys, I have been asked this question several times in training, so I thought I would add it here. This is for estimators, Quantity surveyors, designers, architects and builders
When you create a slab, Either CLT or Concrete, to physically place an order for those items you would order them in cubic measures. However, in some cases, you need to know the square areas of the slab, this is another use for the Surface material in the slab tool.
OR, If you are using PlusDesignBuild you can simply create a Recipe and divide the thickness of the slab by the cubic measures, simply link the recipe to the quantity, this will also deliver a square area from a cubic object and it can be saved and reused for next time. The same applies for liner measure from Square measure, simply create a recipe and link the item to the sq measure and then divide the sq measure by the width and you have a lineal measure. When you get used to using recipes in PlusDesignBuild you will find that you can do anything you need and more. 
I hope that helps. Happy 3D estimating.