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What are the so-called Coordinate descriptors in BUFR? 

The descriptors in Classes 00 through 09 (with 03 and 09 at present reserved for future use) have a special meaning added to them over and above the specific data elements that they describe. They (or the data they represent) "remain in effect until superseded by redefinition" (see Regulation 94.5.3.3). By this is meant that the data in these classes serve as coordinates (in a general sense) for all the following observations. Once you encounter a 0 04 004 descriptor (which describes the "hour"), one must assume that the hour (a time coordinate) applies to all the following observed parameters, until either another 0 04 004 descriptor is encountered or you reach the end of the data subset.

In the above paragraph the "Class" of a descriptor is the X part in F-X-Y (when F=0 or 3).

BUFR Element Descriptor Classes 1 through 9
ClassDefinition
01Identification
02Instrumentation
03Reserved
04Location (Time)
05Location (horizontal – 1)
06Location (horizontal – 2)
07Location (vertical)
08Significance Qualifiers
09Reserved


For further details see section 3.1.2.2 of the WMO BUFR guide: https://rda.ucar.edu/docs/formats/bufr/Layer3BUFRCREX.pdf

Old link: https://www.wmo.int/pages/prog/www/WDM/Guides/BUFRCREX-Layer3-English-only.pdf