YAZ supports facets in the Solr, SRU 2.0 and Z39.50 protocols.
    Like Type-1/RPN, YAZ supports a string notation for specifying
    facets. This notataion maps straight to facets.asn.
    The notation is parsed by function
    yaz_pqf_parse_facet_list defined in header
    yaz/pquery.h.
   
For ZOOM C the facets are specified by option "facets". For yaz-client, the 'facets' command is used.
The grammar of this specification is as follows:
   facet-spec ::= facet-list
   facet-list ::= facet-list ',' attr-spec | attr-spec
   attr-spec ::= attr-spec '@attr' string | '@attr' string
    
    The notation is inspired by PQF. The string following '@attr'
    must not include blanks and is of the form
    type=value,
    where type is an integer and
    value is a string or an integer.
   
There is no formal facets attribute set (it is not given in the protocol by the facets, although it could). The following types apply:
Table 7.4. Facet attributes
| Type | Description | 
|---|---|
| 1 | Field-name. This is often a string, e.g. "Author", "Year", etc. | 
| 2 | Sort order. Value should be an integer. Value 0: count descending (frequency). Value 1: alpha ascending. | 
| 3 | Number of terms requested. | 
| 4 | Start offset (starting from 1) |