Original song usually means that it's original content. In NND this term オリジナル曲 is used. However, usually self-cover works are titled with オリジナル as well. In cases when there is two or more identical songs with different vocalists, the first upload should be original and other songs should covers with Self-cover tag.
Sometimes producers upload "modified versions" of their older songs. 修正版 in the song title usually indicates that it's remastered version. If the original version can not be found (deleted) or it's not in any album, song should be original song type instead of Remaster. Sometimes whole album is made of remastered songs. In this case, mentioning it in the album description or/and using a tag is enough.
Currently arranges should be classified as a remixes.
Sung by human (utaite) / Vocaloid cover / Instrumental cover. Since 17th August Utaite covers has been prohibited (For more info visit VocaDB FB Page). If producer has two or more identical songs with different vocalist, the first upload should be original and other songs should covers with Self-cover tag.
Tag should be only used for instrumental versions of the songs, released within some album. Nicokara (ニコカラ) songs and other non-offical karaoke songs should not be added to database. Karaoke download (piapro links) in the song entries are allowed. If the song is original song and has no vocals, use instrumental tag (Only when the song is part of album in the db). Instrumental songs using Vocaloid/Utau voicabank as a chorus should be Original song type, with a supporting vocalist role.