XML Tutorial/XML Schema/NMTOKENS
The NMTOKENS data type is derived from the NMTOKEN type
<source lang="xml">
The value space is the set of finite-, nonzero-length sequences of values for NMTOKENS datatypes. The lexical space is the set of white-space-separated lists of values for NMTOKENS datatypes.
<xsd:attribute name="degree" type="xsd:NMTOKENS"/>
<title degree="BA MS">Mister</title></source>