<xsd:documentation>A code associated with the relative abundance of the qualifier.
For example, "trace", "common" or "abundant".
This is generally a proprietary code. The interoperable semantics are provided
by the above abundance value. The presumption is that the value range associated with
a code is mapped to a single value. On the other end of an exchange, the single
value is mapped to the range associated with the local codes on that end.</xsd:documentation>