Technically, the Boards we have in the library could be anything you want them to be. 

You can set up different costing/pricing for the different types of boards you want to use in the Settings/Pricing section. Think of Boards as simply 3D shapes which you assign prices, colours, Textiles, Surface Decor and/or Ribbons to.
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For example...
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Lets say you set up the following board pricing options in the Settings/Pricing/Boards section.

You would also set up whatever cost and price you wish to set for each depending on how much you buy and sell them for. Again, it's completely up to you.
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You would then add the desired board from the library and technically could set any of the pricing to any of the boards.
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Say for example, I wanted to sell a thin cardboard, for an internal construction between tiers, I would just add one of the 1mm thick boards and then set the pricing as "Thin Cardboard". Like this...
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Add the board to the canvas...

Select the board and open the Properties Panel...
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Then select the Thin cardboard from the Board Pricing drop down...

As another example, if I wanted to use a cake drum that is 9mm thick, I would add a board to the canvas that is 9mm thick...

Select it, and then open the Properties Panel and select "Cake drum" from the pricing drop down...

So far, in these examples we have just taken care of costing and pricing for the board.
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After this step, you could change the way the board looks with any of the following...

You can do any or all of these at the same time and of course, all of these (with the exception of the fill tool) may also involve costing and pricing.