OVA’s (Original Video Animations), ONA’s (Original Net Animations), and Specials (episodes that are aired similarly as the original broadcast but are aired separately) are all permitted on AniList.
An OVA is a title that released directly to home video via something like VHS, LD, DVD, BD, etc.
For an OVA series to be combined into a single entry, it should be released on a semi-regular schedule and/or should have a shared continuity (see Example 3).
For OVA’s that are attached to a parent series, they should be combined only if they apply to the same parent series (see Example 1).
Yahari Ore no Seishun Love Comedy wa Machigatteiru. OVA, an OVA alongside the first season of the show, was bundled with a limited edition console game. However, another entry would be made if the OVA in question has a tie-in to a specific season, such as Yahari Ore no Seishun Love Comedy wa Machigatteiru. Zoku: Kitto, Onnanoko wa Osatou to Spice to Suteki na Nanika de Dekiteiru.
Kono Subarashii Sekai ni Shukufuku wo! OVA, an OVA alongside the first season of the show, was bundled with a volume of the light novel. However, Kono Subarashii Sekai ni Shukufuku wo! 2 OVA, is its own separate entry as it was tied into the second season of the airing TV show.
The show Kidou Senshi Gundam UC was aired in a set schedule, with each episode following a singular continuity, so it is combined.
An ONA is a type of series that airs online for its original broadcast release.
A work will be labeled special if the production company labels it as such. A special is also a work that is uniquely different than it’s parent series, whether that be a picture drama, a short, a pilot, etc.