SSPX does not have parishes. They have chapels. The difference is important.
It is tempting to be cynical about motives for setting up EF Masses, with good reason. But the reality is, there just are more priests available to say it than before, and recent bishops more favorable than a few decades ago.
Theysay they have supplied jurisdiction, as do the American Catholic Church, and numerous other groups mostly liberal, generally connected through the Old Catholic episcopal lines. There could be other episcopal lines, and justifications for “supplied” jurisdiction.
Whether some, or all, actually have supplied jurisdiction is a matter of contention. Besides the view of the Vatican, the groups themselves might disagree Among themselves about the status of some other allegedly “supplied” group.
The tendency is for each group to say that they are in a special situation, that might not apply to certain others.
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