The unique element

Unique specifications determine which columns of the table are unique in the SQL sense.

<!ATTLIST unique
          key CDATA #REQUIRED
          description CDATA #IMPLIED>

key lists the comma separated identifiers of columns that form the unique constraint.

Here we see that while the primary key of the table Event is id, names are unique.

<table id="Event" writes="full">
    <column id="id" type="int" properties="-superlatives,-value-stands-for-column"/>
    <column id="name" type="text"/>
    <column id="type" default="appearance" type="text" properties="+value-stands-for-column,+value-stands-for-table"/>
    <column id="time" type="timestamp"/>
    <primary_key key="id"/>
    <unique key="name"/>
    <primary_value key="name"/>
</table>