You can check the oto.ini file of any UTAU you have opened for your project by going to Tools(T) > Voice Bank Settings(S).
A CV (single sound) voicebank will have an oto that looks like this.
Note the names in the .wav file column. They're all single sounds, like ちゃ (cha), じぇ(je), etc.
It's also very common to see these names in the alias column.
CV stands for [C]onsonant [V]owel.
か(ka). K is the C, and a is the V.
So to write the word ありがと (arigato) in CV, it'd be...
[あ] [り] [が] [と]
The names in the .wav column of the oto editor can be anything.
It's really the alias (alternate name) that counts.
If the alias column is blank, then UTAU will use the names in the .wav column directly.
UTAU can use either column, though. So it is possible to have romaji wav files (ka.wav, ki.wav, ku.wav, etc) with hiragana aliases (か, き, く, etc) and entering either name into a note will call the same wav file.
For other voicebank types such as VCV, the wav file names are usually in strings like かかきかくけか.wav, and aliased like [- か] [a か] [a き] [i か] [a く] [u け] [e か].
Since VCV is one long wav set up into a series of small segments, you should not use the name in the .wav column in a note. More on VCV below!
VCV (continuous sound) is [V]owel [C]onsonant [V]owel.
Take a close look at the alias column in this VCV voicebank's oto.
VCV works by overlapping the vowel from the next note into the vowel of the previous note.
ありがと (arigato) becomes
[- あ] [a り] [i が] [a と]
We use - before the first あ because it comes after a rest.