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)
What would be the natural way to group requests?
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.