You can use any prefix as long as it belongs to the schema namespace
You specify the schema declaration in the schema file. This is the root elemnt of that document.
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
Now the above declaration can also be
<myPrefix:schema xmlns:myPrefix="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
and then say
<myPrefix:element name='firstName' type='myPrefix:string'/>
But 'xs' or 'xsd' makes more sense than 'myPrefix'
Hope this helps
Hari