The data source code is used in the OBJDESC.ISOURCE record field. It specifies the source of the data. This might refer to the site which generated a data set, but that site might have obtained the data from elsewhere. It might refer to the task which generated the data, but datasets created by two tasks which process the same data differently might still be considered to have the same data source. An instrument which took the data may be considered its source, or the platform from which the instrument operated. The idea of ``data source,'' then, is fairly flexible and open to interpretation; it is therefore locally defined.
These codes are integer values at least 32 bits long.
An example may help in understanding the use of these codes. At Site 1 (NASA/GSFC Chemistry and Dynamics Branch), data are received from the National Meteorological Center (NMC) in their packed format. The data are then unpacked and put into the df format. The site identifier in the AUDIT.TREE record would have one node containing the site identifier for the NASA branch (1) and the task identifier for converting NMC packed data to the df format (16777216). The data source code would be associated with NMC (65536), since they provided the data.
In a second example, data are gathered by scientists at the same site for stratospheric aircraft missions. The site would again be the same (1), but the task of putting these data in the df format would be different from the first one (33554432). The data source would also be different and would indicate the NASA aircraft mission (512).