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.
Note: even though this guide talks mostly about "Vocaloid", almost all of the rules apply to UTAU, CeVIO and other related voice synthesizers as well.
Original songs ("オリジナル曲" in Japanese) are songs that are completely original content and aren't based on other songs. Original song is the version of the song that was published first, later versions are considered as derivatives regardless of who made them.
As long as the above rule is true, the original song type takes precedence over all other types, with the exception of Drama PV (see below).
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.
Sometimes the producer might publish multiple different versions of a song, usually with different vocals or different lengths, calling all of them originals. The songs might be published in different media, for example an album song vs. NicoNicoDouga upload. To avoid misunderstandings, as a rule of thumb the version that was published first, regardless of medium, should be considered the original, and all others are derivatives. There are exceptions to this rule, for example if the earlier version is officially labeled a cover or if it's commonly agreed that way.
Extended ("long") versions of songs should generally be separate from the original song entry on VocaDB, unless the short version was intended to be only a demo, in which case it can safely be replaced by the longer version. Extended versions should generally use the remix type.
Instrumental song that is an original, meaning it's not an instrumental version 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.
Sometimes the artist might release multiple versions of the song simultaneously. In this case the primary tie breaker is artist's intention (which version was intended to be the "original"?). If the primary tie breaker is ambiguous or unknown, the second tie breaker is choosing the Vocaloid song over other versions (for example if the other version is sung by a human vocalist).
Remaster ("修正版" or "リマスター版" in Japanese) means an improved/remastered version of a song. Sometimes artists may replace the original song with a remastered version, even deleting the original.
Sometimes the artist remasters the song for an album release. Quite often album versions of songs are remasters, but this is not always the case.
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. Strictly speaking, as the name implies, a remastered version should differ from the original only in terms of mastering, meaning the original recording stays the same. In the Vocaloid world this term is often interpreted more loosely, and is not always limited to just mastering. Still, if the new version is noticeably different from the original, the remaster song type should *not* be used.
cover: "a recording or performance of a song previously recorded by another performer."
Cover is a re-recorded version of a song, usually by an artist other than the original. 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.
Sometimes the original artist re-releases the song with new vocals. The cover song type should be used in this case, with the self-cover tag.
remix: "a variant of an original recording (as of a song) made by rearranging or adding to the original."
Remix is an edited version of the song, usually made to sound quite different. The remix is often different musical genre from the original. 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.
Extended (long) or shortened versions of songs, if done by the original artist, should primarily be classified as remixes.
All other things being equal, try to figure out what the intention of the artist was when they made the song.
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.
The music PV type is used for remarkable PVs of original songs. By creating a separate entry for the PV the animator and other people involved can be credited accordingly, and things like the MMD models used can be indicated with tags. Sometimes the PV might be more well known than the originally published version of the song, for example the Sweet Devil PV by wakamura and Love is War.
Like with most other song types, music PVs should always be derived versions. If the song was originally uploaded/published with the PV, that PV should be included with the original song entry.
Generally, only official PVs (PVs what were commissioned by the song artist) should be added to the same entry as the original song. Any major derivative PVs made by unrelated artists should have their own (Music PV) entry, regardless of whether the song was originally published with a PV or not.
If the song was originally published without a PV, for example on an album, and an official PV is released afterwards, usually there is no need to create a separate entry for the PV, the PV can be added to the original song entry. PVs with minor differences such as model changes can be added to the original entry as well, with the "other" type.
Drama PV is an animation where the main focus is story instead of music. Most commonly these are made with the MMD software ("MMD Dramas").
Drama PVs don't need to contain any singing, or even talking, but in order to be accepted on VocaDB they should have a Vocaloid character in a major role, or be in some other way strongly related to Vocaloid (or UTAU).
Note that unlike with the other song types, the drama PV type should always be used when a PV is considered drama. This is because the other song types are used to represent music, while the content of drama PVs is fundamentally different. Drama PVs are almost always originals, there are very few derived dramas.
Mashup is a song that is made by blending two or more songs together.
Simple mashups where two songs are simply mixed together are discouraged on VocaDB, unless they're well-known or present on an album. These songs are allowed to some extent, but like with instrumental songs, try to keep the number low and quality high enough.Medley is a special case of mashup where the songs are primarily played one after another, possibly with some overlap. The mashup type is to be used for medleys as well, except in rare cases where the medley is actually an original song, in which case the original song type should be used. Remember to add the medley tag.