[Clarification] Rule for serializing a simple value

The rule for serializing a simple value is,

"A simple value is represented as character data, that is, without any
subelements. Every simple value must have a type that is either listed in
the XML Schemas Specification, part 2 [11] or whose source type is listed
therein (see also section 5.2)" [1]

I have two questions on this rule. I will greatly appreciate if this is
clarified in the spec.

(a) What is a source type?
(b) And, where is this source type listed?

[1] http://www.w3.org/TR/soap12/#_Toc478383513

Regards, Asir