This documentation is adapted from an extract of the WMO Manual on Codes, no. 306.
The columns from CB1 to CB4 are used to report a number of different layers or masses of cloud. The number of columns/groups shall in the absence of Cumulonimbus clouds not exceed 3. Cumulonimbus clouds, when observed, shall always be reported, so that the total number of groups used can be four.
The selection of layers (masses) to be reported shall be made in accordance with the following criteria:
The order of reporting the groups shall always be from lower to higher levels.
In determining the cloud amounts to be reported for individual layers or masses in the 8-group, the observer shall estimate, by taking into consideration the evolution of the sky, the cloud amounts of each layer or mass at the different levels, as if no other clouds existed.
This information is required from land stations and fixed ship stations, mainly in the tropics.