As Manfred said, the Substrate Classes (Class 0,1,2 & K) account for the differences you are talking about between materials. WUFI Bio doesn't do any "smart" or "automatic" interpretation based on the material, it is strictly what substrate class you select that dictates the LIM curve.
For more info, the WUFI Bio help file has a link to the PhD thesis that helped to develop the program. The link is: http://www.wufi.de/ibp/publikationen/di ... tion_e.pdf
It's actually quite a well written thesis and explains LIM curves in a fair bit of detail.
Essentially, every species of mould has a different LIM curve and on top of that they would each have a different curve for each different substrate. What WUFI Bio does is take the LIM curves for all the moulds most likely to grow on building materials and combined them. It has then broadly groups together the different types of substrates based on how easily mould can eat them. Therefore, Substrate Class 2, for example, represents the lowest threshold for all the moulds likely to grow in building based on how well they grow (on average) on biodegradable materials.
Hope that helps!