A Mobility projects can have multiple configuration base on different platforms. In one configuration some sources may needed which maybe unnecessary for another configuration. At this moment we can exclude specific sources from a specific configuration by deselecting them from "Project Properties->Build->Sources Filtering". Everything works ok except the 'Project Tree' shows compile error sign on filtered out (unrelated) sources! 
Lets say I have 2 configurations 'ConfA' and 'ConfB', from ConfA I filtered out the source 'X.java' as it is just necessary for 'ConfB'. Now, while I am working on the 'ConfA' I don't need to be concern about any error on that 'X.java' file. In fact it would be better if that X.java file were not visible in the project tree when 'ConfA' is active or at least no error sing (red [!]) on the project tree. This is kind of annoying.