A question about semantics from the standard library's documentation

[Aseem Bansal]

In Python 3.3.2 documentation on the Python Standard library's introduction these sentences are given

"For these types, the Python language core defines the form of literals and places some constraints on their semantics, but does not fully define the semantics. (On the other hand, the language core does define syntactic properties like the spelling and priorities of operators.)"

That got me confused. What is defined by the language and what is not? 
Can someone give me an example about what this means?