The Herald will announce on either say, group chat (party, arena, or raid), or raid warning the following items when they are being created or cast:
Scrapbot
Field Repair Bot 74A
Field Repair Bot 110G
Jeeves
Goblin BBQ
Broiled Dragon Feast
Seafood Magnifique Feast
Cauldron of Battle
Big Cauldron of Battle
Ritual of Refreshment (Mage table)
Ritual of Souls (Lock Cookies)
Ritual of Summoning (Lock Portal)
MOLL-E (mailbox)
Mass Resurrection
Mobile Banking
Banquet of the Brew
Banquet of the Grill
Banquet of the Oven
Banquet of the Pot
Banquet of the Steamer
Banquet of the Wok
Great Banquet of the Brew
Great Banquet of the Grill
Great Banquet of the Oven
Great Banquet of the Pot
Great Banquet of the Steamer
Great Banquet of the Wok
Great Pandaren Banquet
Pandaren Banquet

/herald will provide details on how to change the settings. The default is set to says. If you set it for Raid Warning, it will automatically default to announcing on group if you are not at least a raid assistant.

As a note, Herald only functions when you are within an instance for a raid or dungeon.