If making a video for television, R 128 in Europe says the total program (meaning all audio tracks summed together) should have an integrated volume of -23 LUFS. This is the “I” bar in the Shotcut audio scope. A/85 in the United States says the dialog track should be -24 LUFS, then provides some latitude for the music and sound effects to be whatever sounds good around the dialog. I vastly prefer mixing to A/85 because it’s easier to hit the target. I even mix home videos to A/85 for its ease and ample headroom.
If mixing for YouTube, their reference level is -14 LUFS, but of course they accept any volume level you send. However, AES has noted and I can confirm from live streaming complaints that volume levels below -16 LUFS are not loud enough to be heard well on the tiny internal speakers of cell phones. So mix audio between -16 LUFS and -14 LUFS if cell phone users are the target audience. A good option is to be like A/85 but put the dialog track at -15 LUFS instead of -24. Any background music added will put the overall volume level at the -14 reference level.
Lastly, don’t be like the audio CD guys or advertisement producers that mix to -8 dB/LUFS thinking they can be louder than everyone else to get attention and sound better. YouTube (and TV and iTunes and everyone else for that matter) will scan the average volume of your audio, and if it’s above their reference level, they will turn your audio down automatically to match their reference level. The end result will actually sound quieter than everyone else due to less dynamic range (unused headroom) after the turn-down. So it pays big-time to not exceed your platform’s reference level. It also pays to not fall too far below their reference level, because they will not turn up your audio to meet the reference level if you submit a super-quiet file. You will simply sound quieter than everyone else, which may be an annoyance to some viewers.