C++ omiting `noexcept` specifier versus `noexcept(false)`, what is their precise meaning?

If I mark a function as noexcept(false), or any other expression which evaluates to false, what does it means? (1) am I ensuring to the compiler that the function can throw an exception?, (2) or am I ensuring nothing about whether it can throw exceptions or not?

And lastly, if I omit the noexcept specifier, it is equivalent to noexcept(false), or only equivalent to the (2)nd meaning stated above?