It's a mid-level infallibility
It is the first time I have ever heard of the notion of "mid-level" infallibility.
Something is either free from error or it is not. If the mere possibility of error do exist, then that something is NOT infallible. When the Church declares someone to be saint then it must be true that the person is in Heaven, at least since the date of canonization.
Beatifications are not infallible, for example. They lack the note of universality. It is within the realm of possibility (not likely, but still possible) that a Decree of Beatification of someone is erroneous. In the case of Canonizations of Saints, however, the Holy See is infallible. This means, absolutely free from all error.