In reality nobody knows, it's very a very complex question that has been barely explored in court.
In practice everybody acts under the assumption that the GPL affects all code that is statically linked together, but nothing else. Some people extend the interpretation to dynamic linking, but consensus is that the GPL does not affect code interoperating over other interfaces like web APIs.
In practice everybody acts under the assumption that the GPL affects all code that is statically linked together, but nothing else. Some people extend the interpretation to dynamic linking, but consensus is that the GPL does not affect code interoperating over other interfaces like web APIs.