+ The <emphasis>syntax</emphasis> of Z39.50/SRU queries is
+ well known to any client, but the specific
+ <emphasis>semantics</emphasis> - taking into account a
+ particular servers functionalities and abilities - must be
+ discovered from case to case. Enters the
+ <literal>explain</literal> operation, which provides the means
+ for learning which
+ <emphasis>fields</emphasis> (also called
+ <emphasis>indexes</emphasis> or <emphasis>access points</emphasis>
+ are provided, which default parameter the server uses, which
+ retrieve document formats are defined, and which specific parts
+ of the general query model are supported.
+ </para>
+ <para>
+ The Z39.50 embeddes the <literal>explain</literal> operation
+ by perfoming a
+ <literal>search</literal> in the magic
+ <literal>IR-Explain-1</literal> database;
+ see <xref linkend="querymodel-exp1"/>.
+ </para>
+ <para>
+ In SRU, <literal>explain</literal> is an entirely seperate
+ operation, which returns an <literal>Zeerex
+ XML</literal> record according to the
+ structure defined by the protocol.
+ </para>
+ <para>
+ In both cases, the information gathered through
+ <literal>explain</literal> operations can be used to
+ auto-configure a client user interface to the servers
+ capabilities.