The ()-surrounded parts are optional. If no seed is specified, a random seed is used instead. When worldname_environment is used, the environment type is appended to the world name, while with worldname/environment it will not append it to the name. Appending the world environment to the name is recommended, since it eases loading the worlds if no configuration is available for it yet. Not specifying an environment means that the NORMAL environment will be used. Bear in mind that the Bukkit 'Environment' and 'Worldtype' are merged as one in MyWorlds. To have a 'nether flatworld', you use 'nether_flat' as environment. Note that most combinations are not used by Bukkit at all and are a bit useless, they are only useful if you have a modded server such as MCPC+ where these custom options are used.