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To start filtering click on the 
icon in the toolbar. This brings up the BUFR filter editor. Having defined the option options and run the filter two tabs will appear in the main interface:
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The message/subset filter itself is split into two parts. The first part defines the options purely using BUFR header data:
while the second part features the options that require access to the data section (i.e. the messages have to be expanded) :
Extracting data values
The data extraction options are all grouped into one big section. By default data extraction is disabled. To enable we need to set option Extract values to On.