BUFR messages are composed of sections. The sections encode the metadata as well as the data itself.

A BUFR message is composed of six sections, numbered zero through five.

• Sections 0, 1 and 5 contain static metadata, mostly for message identification.
• Section 2 is optional; if used, it may contain arbitrary data in any form wished for by the creator of the message (this is only advisable for local use).
• Section 3 contains a sequence of so-called descriptors that define the form and contents of the BUFR data product.
• Section 4 is a bit-stream containing the message's core data values as laid out by Section 3

The metadata (the headers) which describe the structure of the data are always available. Therefore the keys in these sections are accessible as soon as you get a message handle (e.g. via the Python function codes_bufr_new_from_file())
To access the data section, you need a special directive which instructs ecCodes to do the actual unpacking and decoding of all the data elements. This is done by setting the special key unpack to 1.

Here is an example of decoding only the header of a BUFR message.
And here we have an example of decoding all the data (header + data).

A good way of seeing the contents of two parts is via the BUFR Validator. Load your favourite BUFR file there and click Validate and you will see two top-level entries: Header and Data. Expand each one to reveal the ecCodes BUFR keys and their respective values. To decode the keys under Data requires the setting of the above-mentioned "unpack" key.

For further details, please consult the eLearning resource Introduction to BUFR decoding with ecCodes (login needed to access content on Learning Platform).

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