Relationship between office:string-value and content of table:table-cell

A table cell (inside a table row inside a table inside a spreadsheet
document) can have both content and a value, as in:

<table:table-cell office:value-type="string" office:string-value="Hello
World"><text><text:span text:style-name="auto_char_italic">Test
String</text:span></text></table:table-cell>

It seems to be unclear whether an application should show the string value "Hello World" or the content "Test String".