"When a pope issues a document motu proprio, it means he does so by his own motivation. As Catholic News Service puts in its style guide, “Popes use it to signal a special personal interest in the subject.” The phrase motu proprio does not refer to a document itself, but rather the manner in which a document is released. That is, a document is released motu proprio when the pope wants to change or enact an existing church rule—or when he has something to say that is not in reply to a request or a petition but instead comes from his own interest in a particular matter."
https://www.americamagazine.org/faith/2017/09/21/when-pope-releases-statement-motu-proprio-its-important-why
Do you have a list of what is considered “ex cathedra”?