In a project file you can use wildcards to include all the files in one directory or a nested set of directories as inputs for a build. However, there might be one file in the directory or one directory in a nested set of directories that you do not want to include as input for a build. You can explicitly exclude that file or directory from the list of inputs. There may also be a file in a project that you only want to include under certain conditions. You can explicitly declare the conditions under which a file is included in a build.