Official add-ons

The official add-ons are pwads that can be downloaded from Bethesda.net to be played with the Doom Classic Unity port. Since console manufacturers do not usually allow players to use third-party content unless vetted by the publisher of the game, the official add-ons system allows a selection of modifications to be downloaded and played instead of being limited to the IWADs. Official add-ons are curated to ensure they are playable within the raised limits of Doom Classic, and to avoid potential copyright infringement in their content. Also, the consent of all original authors is mandatory, according to a Doomworld forums by id Software member Mike Rubits (sponge), so that they all have to be contactable by id Software in order to license their work.

Note that "official" in this context refers to provision of the content by id Software. There has been no statement indicating that the modifications should be considered canonical story-wise.

List of official add-ons
The two Final Doom IWADs are available as official add-ons for both The Ultimate Doom and Doom II. This effectively makes Final Doom free for all owners of either Classic Doom titles on Bethesda.net.
 * The Plutonia Experiment
 * TNT: Evilution

Ultimate Doom

 * Double Impact
 * SIGIL

Doom II

 * No Rest for the Living