The importance of a "Terms of Use" (ToU)
UTAU sure is an interesting concept. UTAU are generally considered to be half OC and half software, though with varying emphasis on either side depending on the individual creator. Your own UTAU is your intellectual property and you should have full rights over its usage and distribution.
Of course, I say "should" for a reason—the inherent nature of UTAU is derivative. Voicebanks are typically provided for free and open use to the community without much restriction. The software itself has a few rules, but other than that, individual UTAU are basically left to the wind.
If there are certain instances where you do NOT want your UTAU used or modified, you should write up a Terms of Use! A clear and concise ToU will help you defend your IP and provide you with some degree of legal protection… which of course hopefully won't be needed.
An UTAU ToU should always be distributed in your readme.txt file (included in the UTAU voicebank folder). If it gets to be too long and clutters the readme text, you can always post it on a pastebin page, UTAU Wiki page and/or website and add the link to your readme instead!
Some example UTAU Terms of Use:
• Kikyuune Aiko
• Kyou Hana
• Kasane Teto
Writing the ToU
Some general questions to ask yourself before writing up your ToU:
• What am I ok with?
• What am I NOT ok with?
• Would I want to see my character used like this?
• Would I be ok with my UTAU being used if I leave the community?
• Should I allow my UTAU to be redistributed?
• Am I ok with people making money off my UTAU?
You should use clear and concise language. You don't want to oversimplify, but you don't want to write a bunch of jargon either! If you don't want to become too wordy, bulletpoints are a good way to make basic lists of what you deem to be acceptable and unacceptable situations, and you can always build off these later.
Usage Examples
Some examples of general usage that are commonly limited or prohibited in an UTAU ToU:
• Libel/slander (i.e. making false claims or accusations)
• Hate speech (content which incites violence or hatred)
• Commercial use (i.e. monetizing songs made with the VB, uploading songs to Bandcamp or other distribution sites, commercializing art or products with the character image)
Some examples of software usage that may be limited/prohibited:
• Modifying and/or redistributing wav files
• Repurposing recordings for another UTAU/character
• Modifying and/or redistributing otos (however, limiting this is not recommended)
Some examples of character usage that may be limited/prohibited:
• Violence/gore
• R-18 (pornographic) content
Be aware if your Terms of Use becomes overly strict. While it is fully within your power to do so, don't be surprised if people balk at UTAU that are prohibited from singing a specific song, from being drawn in any clothing other than their default design, or anything else excessively limiting.
Other things to note
Distinguishing between character and voicebank
Are you ok with the voice files being used commercially, but not the character design? Be sure to specify your preferences on commercial usage between the character image AND the voicebank folder.
Contact
Always provide a way for users to contact you if they are unsure of usage or require permission!
However, it would also be wise to include a clause that states that if you cannot be contacted for any reason, to simply not produce the material in question.
Doujin
Doujin works are a tricky thing, and since the UTAU fandom originated in Japan, it's worth noting this distinction. Many UTAU doujin events are held in Japan each year, sometimes overseas UTAU that get noticed make their way into the products offered. It's important to note that "doujin" works are derivative works that are created and sold in small, limited batches and generally only break even / do not generate profit. Doujin is done in the spirit of enjoyment and promotion of the media involved.
It would be wise to note a distinction for "doujin" usage, ESPECIALLY if you promote your UTAU by posting on Japanese sites such as Nicovideo. You never know who might pick up on your VB/character!