It’s no immediate indicator for the intensity but it says a lot of how much muscle fibres you have activated in the corresponding muscle.

Everyone who gets in touch with volume training, will admit that no matter how hard you train a “hard growing” muscle , you will never get the pump effect.
Other muscles can be pumped up very strong: their development goes easy.

Muscles have a kind of “regeneration memory”: if they learned to activate as much muscle fibers as possible – they can remind that any time.
An optimal training activates as many muscle fibres as possible.

But which training is the right one?

Volume training is good for activating many muscle fibres.
But it’s far from effective. Between the sets the muscle fibres get enough rest to relax. So it’s hard to activate new muscle fibres that weren’t in the game before. But the main problem is the relaxation factor.
Because of the many sets, there is a need for longer rest periods. This results in a good muscle gain at the beginning of a training career that comes to a phase of stagnation after a while.

In turn, HIT doesn't use enough fibres when performed in it’s classical style.