Sub-filters appear when you select a particular option within the Record types global filter. They are displayed below the global filters, just to the left of the search results. 

For example, in BCH, if you select Competent National Authority (in the “National records” category), the following sub-filters appear on the left side (scroll down to see them): ‘Free Text’, ‘Administrative functions’, ‘Types of organism(s) under its responsibility’. You can continue to use these sub-filters to further narrow your search for the Competent National Authority(ies) of interest. Each record type has its own specific set of sub-filters. The sub-filters, including the Free Text, apply only to the selected record type.  

You can continue narrowing your search by using the sub-filters only, or a combination of sub-filter and global filters.

Please also TAKE SEARCH TOUR.