When a Standard voice is being recorded, the various samples are all recorded at multiple pitches. Expression groups represent the different pitches and tone variations of recorded samples. By default Synthesizer V Studio will select the most suitable expression group for the notes you have entered, but you also have the option to manually change this. This is useful if you want to force the engine to use soft or falsetto samples, or prevent it from doing so. Pictured below is the list of expression groups included with Genbu.
Since AI voices are based on a machine-generated profile rather than discrete samples, there are no expression groups to pick from and this is a feature specific to Standard voices.
To expand on this explanation using Genbu’s list above, what this means is that when recording the samples for Genbu’s voice database they recorded an extensive list of phonemes and phoneme transitions. This process was not only done at one pitch though, they repeated this entire process at A3, C3, and F4, as well as two additional repetitions at G3 and F4 in which the voice provider used a soft and falsetto tone respectively.
Let’s imagine you input a note at C4 in the piano roll. By default the engine will choose either the “G3Soft” or “F4” sample since those are closest, then pitch-shift it to C4 and stitch it together with the surrounding notes to create smooth transitions.
Since you’re working in a range exactly between two of the expression groups, it’s possible that the engine will not choose the same samples for each note; some may be pitch-shifted upward while others downward, which could cause odd artifacting or an inconsistent tone. Selecting the expression group manually will mean that all of the selected notes use the same set of original samples.
In the case of Haiyi the “A4F” option indicates a set of falsetto samples recorded at A4.