<scalar dataType="xsd;string" title="NickName">carbonRulez</scalar>
The CML schema suggests to use the XML Schema Datatypes, as defined in the XML Schema Part 2: Datatypes specification. For these data types, commonly the xsd prefix is used, as in the above example.
Now, this specification allows restricting the content of the string in many ways. For example, that it defines a year:
<scalar dataType="xsd;year" title="DateOfDiscovery">1891</scalar>
It defines fourteen primitive datatypes: string, boolean, float, double, decimal, timeDuration, recurringDuration, binary, uriReference, ID, IDREF, ENTITY, NOTATION, and QName. Of these, the first five have obvious meaning. The specification also defines a number of derived datatypes. which include these useful types: language, integer, positiveInteger, negativeInteger, data, month and year.
The CML schema also defines a large set of custom data types. For example, it defines an elementTypeType which derives from the xsd:string and allows only the one/two/three character element name abbreviations ("C", "N", etc), and Dummy, Du and R.
The use of datatypes in XML Schema's is really powerful, and the CML schema allows to define custom datatypes if needed. But I'll save that for a later time.