They involve fighting a pre-scripted battle against specific enemies in a specific battle map with an introductory cutscene. In the campaign, set piece battles usually come as part of a quest for a legendary lord to gain a legendary item. In campaign, these battles can only be accessed by moving to a specific location on the campaign map after completing all the prerequisites (players also have the option of instantly teleporting the legendary lord's army to the location at the cost of money). However set piece battles can also be accessed as a separate game mode, from the main game menu.
Factions encountered in quest battles can use many names (ie. The Messengers of Middenheim) but they still use the units of a base faction or Rogue Army. The crest that represents the faction in most cases is actually the Rebel crest, even if fighting a faction that would normally have it's own crest like Naggarond. In this way, the Rebel Crest is used as a more general representation of the faction instead of specifically representing rebels. Oddly, the Dwarfs are sometimes represented by their Rebel crest, and sometimes represented by a white version of their default crest.
See the following pages: