Looking up what the br, -, and + symbols are called in Synthesizer V. I found this in the official manual:

https://synthesizerv.com/manual/entering_lyrics.htm

Syllable Break
Use plus sign to break down a multi-syllable word. In this example, the first note says "birth" and the following says "day".

Legato
Use minus sign to mark the continuation of the last sung vowel.

This in an unofficial manual:

https://manual.synthv.info/quickstart/entering-lyrics/#adding-breaths

Breath notes can be added by entering br as the note's lyric (AI singers only).

I also found these symbols on Dreamtonic's website under a section titled "Synthesizer V Studio
Phoneme Reference": https://dreamtonics.com/svstudio-resources/

Looks like people interchangeably use the terms "note" and "phoneme" for it, but I'll stick with "phoneme" since that's what the reference is called.