This event represents the "heart" of an XML document: the character data between element start and end tags. The parser passes the address and length of the text containing the this data, which usually consists of a single string only, even if it is split across lines:
<element1>This character content is split across two lines</element1>
The parser also passes a flag byte which indicates if the next event will provide additional characters that form part of the content. This can be true when there is a lot of data between the start and end tags.
The parser also uses the content_characters event to pass the text of CDATA sections to the application.