In general it is organised, as a huge tree, with the indentation below, showing different levels down that tree:

• origin and model (eg SHMI, Atmospheric model etc)
  • type of data (analysis, forecast, etc)
    • year
      
      • month
        • type of level (model level, pressure level, surface etc) 
          • dates, times, steps (for forecast type), levels (for non surface levels), parameters, members (for ensembles)
            

The idea is to request as much data as possible from the same tape file . The natural way to group requests would be:
all parameters, all levels, all time-steps for all dates of a month

Note: 'all' means 'all' that the user wants. It doesn't have to be all parameters.