I will try to compile a list for the flags available for UTAU's default resampler
Please, notice that there are some flags which I don't know exaclty their effects, so I used other sources for information on them, but it may be a bit vague:
a: it affects the fixed region (pink area on oto.ini), by stretching it or compressing it. The default value is 100, so lower than that stretches, and higher than that compresses. Be careful, because the consonant itself will be displaced. Usable range is undefined.
b: breathness flag, is applied after synthesis, so it is not affected by frequency filter flags. Default value: 0; usable range: 0~100.
B (or BRE): breathness flag, applied before synthesis. Default value: 50; usable range: 0~100.
c: value of C flag before formant filter adaptation. Default value: 50; usable range: 0~100.
C: high frequencies filter. Cuts high frequencies. Default value: 0; Usable range: 0~100.
D: mid frequencies filter. Cuts middle frequencis. Default value: 0; Usable range: 0~100.
E: low frequencies filter. Cuts low frequencies. Default value: 0; Usable range: 0~100.
F: formant filter strength tuning. Default value: 3; usable range: 0~?
g: gender factor flag. Positive values will deepen the voice, while negative ones will make it more childish, or effeminate. Default value: 0; usable range: -100~100
G: forces resampler to render .frq files before playing. This makes the synthesis process slower. Flag used without value.
H: low pass filter. Emphasizes low tones, and cuts higher ones. Default value: 0; usable range: 0~99.
h: low pass filter. Emphasizes higher tones, and cuts low ones. Default value: 0; usable range: 0~99.
L: Sets a fixed frequency for F flag. Default value: none; usable range: 0~?
N: turns formant filter off. Flag used without value.
P: Peaks compressor. Makes the overall volume of the voice more balanced, but sometimes causes slurs. Default value: 86; Usable range: 0~100.
t: pitch fine tuning. Shifts the pitch by 10 cents for each point added. Default value: 0; usable range: -9~9
W: Produces a robotic and metallic voice. Tends to highly damage the consonants. Flag used without value.
x: affects the voice according to the distance to the recording note. Higher notes make the voice brighter, while low notes make the voice more muffled. High values of this flag tend to make the voice very robotic. Negative values affect the formant. Default value: 0; usable range: -100~100.
Y: breathness flag. Controls the breath component of the "vowel" part of a note. Default value: 100; usable range: 0~100.
Please, notice that there are some flags which I don't know exaclty their effects, so I used other sources for information on them, but it may be a bit vague:
a: it affects the fixed region (pink area on oto.ini), by stretching it or compressing it. The default value is 100, so lower than that stretches, and higher than that compresses. Be careful, because the consonant itself will be displaced. Usable range is undefined.
b: breathness flag, is applied after synthesis, so it is not affected by frequency filter flags. Default value: 0; usable range: 0~100.
B (or BRE): breathness flag, applied before synthesis. Default value: 50; usable range: 0~100.
c: value of C flag before formant filter adaptation. Default value: 50; usable range: 0~100.
C: high frequencies filter. Cuts high frequencies. Default value: 0; Usable range: 0~100.
D: mid frequencies filter. Cuts middle frequencis. Default value: 0; Usable range: 0~100.
E: low frequencies filter. Cuts low frequencies. Default value: 0; Usable range: 0~100.
F: formant filter strength tuning. Default value: 3; usable range: 0~?
g: gender factor flag. Positive values will deepen the voice, while negative ones will make it more childish, or effeminate. Default value: 0; usable range: -100~100
G: forces resampler to render .frq files before playing. This makes the synthesis process slower. Flag used without value.
H: low pass filter. Emphasizes low tones, and cuts higher ones. Default value: 0; usable range: 0~99.
h: low pass filter. Emphasizes higher tones, and cuts low ones. Default value: 0; usable range: 0~99.
L: Sets a fixed frequency for F flag. Default value: none; usable range: 0~?
N: turns formant filter off. Flag used without value.
P: Peaks compressor. Makes the overall volume of the voice more balanced, but sometimes causes slurs. Default value: 86; Usable range: 0~100.
t: pitch fine tuning. Shifts the pitch by 10 cents for each point added. Default value: 0; usable range: -9~9
W: Produces a robotic and metallic voice. Tends to highly damage the consonants. Flag used without value.
x: affects the voice according to the distance to the recording note. Higher notes make the voice brighter, while low notes make the voice more muffled. High values of this flag tend to make the voice very robotic. Negative values affect the formant. Default value: 0; usable range: -100~100.
Y: breathness flag. Controls the breath component of the "vowel" part of a note. Default value: 100; usable range: 0~100.
