The database tag has attr tags for subtags. When inheriting from a class, the database tag will merge the two attribute trees into one. Attributes at the same level in the tree which have the same type and value will override previously inherited attributes. One for each distinct database that the daemon can access. The tag can contain other tags.
Substitution is done by looking up the tag having the corresponding name attribute.
Mapping of z39.50 terms and operators to target expressions.