Any disadvantages for std::atomic_flag not providing load or store operations? (Spin-lock example)

Comparing a std::atomic_flag to an std::atomic_bool, it seems to me that a std::atomic_flag just has a simpler interface. It provides only testing+setting and clearing the flag while an std::atomic_bool also provides overloads to several operators.

One question of mine is about terminology: What is meant by "load or store operations"? Does it mean that it is not possible to arbitrarily read and modify a std::atomic_flag's value?

Furthermore, I am wondering, could a std::atomic_bool be faster when being used for a spin-lock? It seems to me that an std::atomic_flag always must read AND write during a spin-lock:

while (my_atomic_flag.test_and_set()); // spin-lock

while an std::atomic_bool would only have to perform a read operation (assuming that the atomic bool is implemented lock-free):

while (my_atomic_bool); // spin-lock

Is an std::atomic_flag strictly more efficient than an std::atomic_bool or could it also be the other way round? What should be used for a spin-lock?