Configures output *.iml file. It's optional because the task should configure it correctly for you (including making sure it is unique in the multi-module build). If you really need to change the output file name (or the module name) it is much easier to do it via the moduleName property!

Please refer to documentation on moduleName property. In IntelliJ IDEA the module name is the same as the name of the *.iml file.