It is today possible with the IRAM-30 m to map a square degree field in CO (2-1). As an order of magnitude, this gives a final spectra cube of about spectra with a slightly oversampling of . An observer who has just spent a few hours observing the same source in OTF mode may want to process all the dumped spectra at once even though they do not belong to the same scan.
The LOAD command gathers all the individual spectra currently in the index as a 2D array for future work, in particular visualization. This requires that all the spectra currently in the index are coherent, i.e. same source name, same line name and above all exactly the same frequency sampling. The latter can easily be achieved just by resampling. No checking is currently done about this, i.e. this is currently the responsibility of the observer to ensure a coherent frequency axis. The /INDEX option of commands like PLOT or BASE modifies there behavior to directly work on the 2D array defined with the LOAD command.
Nota Bene: The STRIP command which is producing a Velocity-Position plots, is obsolescent. Indeed the same functionality can be achived by the combination LOAD; PLOT /INDEX if the index is correctly choosen. And the LOAD command is much more powerful.