XML Tutorial/XML Schema/NMTOKENS

The NMTOKENS data type is derived from the NMTOKEN type

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.
<!-- schema -->
<xsd:attribute name="degree" type="xsd:NMTOKENS"/>
<!-- instance document -->
<title degree="BA MS">Mister</title>