How are "fields" defined in bibliometrics.

[Jacques Wainer]

A recent post by Loet Leydesdorff <loet at LEYDESDORFF.NET> writes:
| Fractional citation counting thoroughly solves the issue of
| normalization for differences in citation behavior among fields. This

and

| The claim of validity for the SNIP
| indicator is hollow because the normalizations are based on field
| classifications which are not valid. 

How are fields defined? I am interested in sub-areas of Computer Science
but I cannot find any definition of which are said sub-areas, and I
realized that probably the same problem was solved a a higher lavel,
when the different fields fo science were defined.

thanks

jacques wainer