Ships put apart, WMO identification numbers are allocated to ocean platforms reporting on the Global Telecommunication System (GTS). WMO numbers are allocated depending upon deployment area, and platform type (i.e. drifting buoys, moored buoys, ocean reference sites, and profiling floats). The rules for the allocation of WMO numbers are described on the WMO website. In alphanumeric codes, platform identifiers were 5-digit numbers (A1bwnnn). In BUFR, they are split in three elements: A1, bw and nnnnn. 'nnnnn' may be now up to 99999. So, the complete identifier of a platform (combination of the three BUFR elements) must now be consirered as a 7-digit number:
Platform identifier = A1 * 1000000 + bw * 100000 + nnnnn
In general(1), identifiers with 5-digit numbers may be converted into 7-digit numbers by inserting '00' in 3rd and 4th position. For instance, 5-digit platform ID 64787 is equivalent to 7-digit ID 6400787. This is a unique platform. The opposite conversion is only feasible when zeros are present in 3rd and 4th positions. Due to the lack of available 5-digit identifiers at WMO, observation data are already sent onto the GTS in BUFR only. 7-digit identifiers having another value than '00' in 3rd and 4th position cannot be sent in FM18 BUOY code.
Exceptions exist, most especially in areas around Antarctica. Complete rules are described on WMO website.