For questions specific to the Siege of Terra series of books set in the Warhammer 40k universe. Always use in conjunction with the [warhammer40k] tag.
The Horus Heresy series of books finished with the publication of The Buried Dagger. Black Library announced that subsequent books would be focused on the battle for Sol and come under the Siege of Terra banner. Any questions related specifically to this series of books should have this tag.