group symbols
About flatten or Hierarcaly schematics is another issue and has nothing to do with rooms.
In a hierarcy You have a "upper" main sheet that points to "lower" sheets. Inter connections in these lower sheets has to be done at top sheet, eg in a bus struckture. Some nets if defined correct, do not need this (power and grounds).
Those sheets could be either in one dsn file or seperated in to many. The last have one advantage when You have a sheet You would like to reuse.