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

1) Deterministic data (stream oper):

• origin and model
  
  • 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

2) Ensemble data (stream enda):

• origin and model
  • 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 items on the lowest level in the trees below are archived on the same tape

The idea is to request as much data as possible from the same tape file (all selectable items listed in the web listboxes)  

 The natural way to group requests would be for example for the deterministic data:
all dates, times, steps, levels and parameters of a given month

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