Rewrote last paragraph of Sect II,
 trying to resolve the following problems with v5:

Two things are mixed:
  - A raw data file may contain some entries that are not needed
    for data analysis (but maybe valuable for long-term documentation
    of the used instrument, or for organisational purposes: for
    instance, the names of the experimentalists clearly belong into
    a raw data file, but are not needed for data analysis).
  - NeXus foresees many different types of fields, of which any
    given instrument only uses a subset.
The numbers 100 and 20 are too arbitrary. The paragraph is
  not yet convincing as a rationale for application definitions.