The most complete and up-to-date official list of AL-legal works is found within the ALCC, or Adventurers League Content Catalogue (available for free as part of the D&D Adventurers League Player & DM Pack).
The type of work you are referring to is generally known as Community-Created Content, or CCC. The details of how an adventure or supplement becomes a CCC are detailed on p. 67 of the ALCC:
The CCC adventure design process is fairly open in that the AL administrators provide review for major plot and thematic elements but only rarely provide full critique of the submission. Some things to keep in mind during your creation process:
- Adventures must be appropriate for all audiences; some parental guidance is okay
- CCC adventures aren’t eligible for some DM Rewards
- Adventures must be 2 or 4 hours in duration
- Adventures must be based in the greater Moonsea area; refer to the image below for a general sense of available places
The adventures currently accepted as CCC are listed thereafter on the CCC Adventure List (currently pp. 68-76 of the ALCC). For example, the Border Kingdoms Region Guide supplement is listed on p. 71, with the code 'CCC-GHC-BORDER'.
In the case of the Guild Adept works, those seem to (thus far) have always been "Hard Cover" works, or DDHC. The ALCC offers a complete listing of these works by Season (see complete document, too many pages to list succinctly). The ALCC offers the following guidance regarding Guild Adepts works on pp. 3, 50:
Also... Guild Adepts! This program serves to highlight some high-performing content contributors from dmsguild.com. Not every submission into the Guild Adept program are valid for Adventurers League play, but every entry is intended to promote the overall seasonal “feel” and flow for its companion hardcover adventure. Guild Adept contributors are selected by Wizards of the Coast directly.
What's a Guild Adept?
A Guild Adept is a highly proficient content creator that was chosen by Wizards of the Coast to provide supplemental material for the hardcover adventure. This content can take many forms, and while not all of that content will be approved for organized play, it can still be used for great home games and inspiration for your players and you, the Dungeon Master! In most cases, Guild Adept content counts as part of the hardcover.
Since DDHC products are part of Seasonal play (i.e. released alongside it instead of ad hoc like CCC), and are reviewed by WoTC's AL team to determine what parts to include or exclude from organized play, they are certainly more vetted than CCC. Whether the level of vetting and testing approaches, meets, or exceeds normal DDAL levels is not covered by any primary source I've found, although there's an official interview about the program (here) that may have more information.
