The official add-ons are third-party PWADs that can be downloaded for free 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. Last but not least, the written legal consent of all involved authors for each and every included asset is mandatory, so that they all have to be contactable by id Software in order to license their work.
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. Thanks to DMAPINFO, most of the curated PWADs are made available to both games.
Note that "official" in this context refers to provision of the content by id Software. There has been no statement indicating that the modifications created by the community should be considered canonical story-wise.
List of official add-ons
|Title Image||Name||Game||Authors||Original release||Made available||Number|
|BTSX Episode 1||Both||Esselfortium et al.||2012/12/10||2020/06/25||8|
|BTSX Episode 2||Both||Esselfortium et al.||2014/09/01||2020/08/14||9|
|Doom Zero||Both||Christopher Golden (DASI-I)||2019/09/30||2020/12/15||11|
|Double Impact||Both||Ralphis & RottKing||2011/11/30||2020/02/13||5|
|No End In Sight||Doom||Emil Brundage, Christopher Lutz, Xaser||2016/11/20||2020/03/31||6|
|No Rest for the Living||Doom II||Nerve Software||2010/05/26||2020/01/09||3|
|REKKR||Both||Matthew Little (Revae) et al.||2018/07/10||2020/09/22||10|
|The Plutonia Experiment||Both||Dario Casali & Milo Casali||1996/06/17||2020/01/09||2|
|TNT: Evilution||Both||Team TNT||1996/06/17||2020/01/09||1|
On PC, the games are downloaded to subfolders of Users\<username>\Saved Games\id Software\DOOM Classic\WADs or Users\<username>\Saved Games\id Software\DOOM 2\WADs. The Saved Games folder exists under the Windows "Users" directory, which typically exists on the C:\ drive, and then under the folder named for the user who played the game when the modifications were downloaded; a full example would appear similar to C:\Users\JohnDoe\Saved Games. On Android, the folders are Android/Data/com.Bethsoft.DOOM/files/WADs and Android/Data/com.Bethsoft.DOOMII/files/WADs. On other platforms, the add-ons folders are not meant to be accessed by the user.
Each folder contains the add-on data WAD as a file named simply after its number (and without extension, so for example instead of plutonia.wad it is simply named 2), four numbered JPG images named after the add-on, typically representing the title screen and three screenshots (e.g. plutonia1.jpg, plutonia2.jpg, plutonia3.jpg, and plutonia4.jpg), and a metadata file containing the add-on's description in English, French, Italian, German, and Spanish, as well as the release data (in YYYY/M/D format), the path to the main data, and the names of the images.