As a general rule, as with everything else on VocaDB, information that is already provided in one way usually does not need to be repeated in other ways.
Original songs ( "オリジナル曲" in Japanese) are songs that are completely original content and aren't based on other songs.
Note that on VocaDB self-covers and self-remixes, made by the artist him/herself, are to be classified as covers and remixes, not "original songs", unlike on NicoNicoDouga.
Instrumental song that is an original, meaning it's not an instrumental versions of some other song, should be classified as original song, not instrumental. Instead, you should indicate that the song is an instrumental by using the instrumental tag.
Remaster ("修正版" in Japanese) means an improved (such as remastered) version of a song. Sometimes artists may replace the original song with a remastered version, even deleting the original.
In order to use the remaster song type the song should sound almost identical to the original, with the general melody and song structure intact.
Remix is an edited version of the song, usually made to sound quite different. The remix might be different musical genre. This song type should also be used even when the remix was made by the original artist. In this case you should add the self-remix tag.
Songs that are called "arrangements" should usually be classified as remixes on VocaDB.
Cover is a re-recorded version of a song. Most commonly in the Vocaloid/UTAU world covers are used to record the song with a different voicebank from the original, keeping the instrumentals intact. Sometimes a cover is another artist's interpretation of the song, in which case it might sound almost identical to the original.
AKA off-vocal or karaoke.
Instrumental song type should be used for one purpose: instrumental versions of songs, when a non-instrumental version with vocals is the original. This means, just like cover and remix, instrumental should always be a derived version.
In most cases Drama PV is animation story made with MMD. In most cases unrelated
music (excluding background music).
Mashup is a song that is made by blending two or more songs together