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), 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.