Not much is editable yet. When run, if the level to load does not exist, it creates a new one with that name, using the default random seed (unless specified otherwise)
if the server is run without any command line arguments, the default settings are:
|command||parameters||description||-i||folder_name||Specifies the level name. Will be found in saves/ folder||-s||random_seed||Specifies the random seed to use for terrain generation. Is used only for new levels.|
Single player levels also load in multiplayer and vice versa. Therefore you can keep creating new levels in single player until you find a world you like, then load that in multiplayer.