For the purposes of this discussion, I assumed it was understood that infallibility with equated with being “with out error”.
The pope is infallible / exercises his infallibility / without error, *only* when he defines a doctrine ex cathedra, this is dogma.
The Church's Magisterium is always "unable to be mistaken," enjoys "immunity from error," and “could by no means commit itself to erroneous teaching,” these are quotes taken from the papal teachings in reply #1.
IOW, unlike the Magisterium, the pope requires an event to be infallible / without error, whereas the Magisterium is always infallible / without error because the Magisterium is not human or humans at all, the magisterium is all the teachings that have always been taught by the Church i.e. "all that has been handed down." It is the Church who is the Authority i.e. "the ordinary teaching authority of the entire Church spread over the whole world" - -
"all that has been handed down as divinely revealed by the ordinary teaching authority of the entire Church spread over the whole world, and which, for this reason, Catholic theologians, with a universal and constant consent, regard as being of the faith." - Pope Pius IX
So to say the magisterium is infallible, although correct it can give the impression that an extraordinary or unusual event is required for it's infallibility, which it isn't, any more than an event is required for the Deposit Of Faith to be infallible. By their very nature they're immune from error - as the popes are quoted as teaching.