The on-line document "format_grib" describes the WMO GRIB format;
the ECMWF version is slightly different.  First of all, ECMWF uses
the older Edition 0 rather than the version which is described, Edition
1;  secondly, the parameter table used by ECMWF has different values for
the variables.  This file lists differences between the WMO GRIB format
described in the document "format_grib", and the actual format used by ECMWF.

==> (Note that ECMWF has changed their GRIB format to Edition 1, as of
July 1993 for ds111.2 and as of January 1993 for ds111.0, ds111.1, and 
ds111.3.  The WMO version applies directly except that the parameter
table is still the ECMWF table rather than the WMO table.)

Section-by-section differences of the ECMWF version from the WMO version:
==> All changes apply to both Edition 1 and Edition 0 except as marked.

SECTION 0: THE INDICATOR SECTION (IS)

In the ECMWF version, this section is only 4 octets (bytes) long, and
consists only of the ASCII word "GRIB".  [EDITION 0 ONLY]

SECTION 1: THE PRODUCT DEFINITION SECTION (PDS)

Octet 4 has the value 0 (Edition number of GRIB specification).
Octet 7 has the value 255 (non-standard grid) for all ECMWF/TOGA
grids.  
Section 1 is always exactly 24 octets in length, rather than the 28-octet
minimum specified by WMO.  [EDITION 0 ONLY]
The tables are all the same EXCEPT for Table 2 (Indicator of Parameter
and Units);  the ECMWF version of this table is in the file parm_table
in this directory.

SECTION 2: GRID DESCRIPTION SECTION (GDS)

This section is specified as optional by WMO, but it is always present
in the ECMWF/TOGA datasets (because the grid type is always 255, 
non-standard).  
Octet 4 contains the number of unused bits at the end of section 2.
Octet 5 is set to zero.

SECTION 3: BIT MAP SECTION (BMS)

This section is specified as optional by WMO, and it is never present
in the ECMWF/TOGA datasets.

SECTION 4: BINARY DATA SECTION (BDS)

If the data are in spherical harmonic coefficients, the real part
of the (0,0) coefficient is stored in the same manner as the reference
value, in octets 12-15;  in this case, the rest of the binary data
starts in octet 16.

SECTION 5: END SECTION

The ECMWF version of GRIB is identical to the WMO version for this section.