Within CLASS , an observation should represent a single observing configuration, e.g. for spectra a single direction observed at a single central frequency with a single spectral resolution and in one polarization only. All versions of a given observation represent different stages of the data reduction. Each time the observation is modified (using UPDATE or WRITE) the version number increases. In principle, in CLASS , only the last version of a given observation is relevant. Previous versions are never deleted, with the exception of UPDATE command which overwrites the current version, so you can go back to previous stages of reduction in case of big mistakes.
Provided you respect this use of the version number, data reduction can be largely automated. Failing to do so, i.e. using the same observation number for very different things, implies that you have to remember yourself which version corresponds to which configuration. This was unfortunately the case for previous releases of the OBServing program on the 30-m (See CAL manual), but should be solved in the new version for which Scan numbers and Observations numbers are different. Note that, for bookkeeping purpose, CLASS\ keeps also track of a Scan number, which can be used as a selection criterium.