conclusion:
While papal bulls always used to bear a metal seal, they now do so only on the most solemn occasions. A papal bull is today the most formal type of public decree in the name of the pope, who is bishop, first among equals.

This is an “episcopal bull”: https://en.wikipedia.org/wiki/Correctio_filialis_de_haeresibus_propagatis
This is an episcopal bull broadcasted:
http://www.catholicworldreport.com/2017/11/01/fr-thomas-g-weinandy-explains-his-critical-letter-to-pope-francis/