How do I select the type of classification? I'd like to know for example when to use the HS2002 or SITC Rev. 1?

In general, the rule of thumb in using the classifications is : use older classifications (SITC, Rev.1 and SITC,Rev.2) to obtain long time-series and use recent classifications (HS-2002 and HS-1996) for more detailed information.

The drawback of use of SITC, Revision 1, is that a lot of modern technology was not available at the time of the introduction of this classification (around 1960), which means for instance that it is difficult to find the classification for PCs, fax machines, color printers or mobile telephones (to name a few).

The newer classifications (like HS-2002) will have more clearly defined commodities for those mentioned items, but have been in use only recently, which means that only a few years  of data can be found for that classification.