I have recently invested in the range of UAD2 plug-ins, but am afraid I am not sure how to read the VU meters correctly. I am fine with the VU meter showing gain reduction on a compressor, but when it comes to the output reading — for example, +4dB or +10dB on the same compressor — I am not sure for what I am aiming for, output-wise? Am I right in thinking that the nominal operating level should be averaging at around 0VU?Also, I think I need to catch up on my dBus and my dBFSs. If my aim is to have the average level around or slightly above 0VU, I take it that going into the red is OK, as long as the average level is around 0VU. I think I remember reading that VU meters didn't respond to high transients very well, hence going into the red, so that would make sense.Via SOS web site
SOS Technical Editor Hugh Robjohns replies: The VU meter (and the PPM) are analogue tools designed for the analogue world. They indicate signal levels around the nominal operating level and they don't show the headroom margin at all.
0VU is the nominal operating level and, in the analogue world, that is usually (but not always) +4dBu. Most decent analogue equipment clips at about +24dBu. This means that when signals are averaging around the 0VU point there is about 20dB of headroom to capture the fast transient peaks that the meter can't show. 
A nominal operating level would be 0VU, which normally equates to +4dBu in the analogue world. As most good analogue equipment clips at around +24dBu, there is usually about 20dB of headroom to capture fast transient peaks that the meter can't show when the signal is averaging at around 0VU.
Digital peak meters, in contrast, do show (most) transient peaks and do show the headroom margin. The clipping point is always at 0dBFS, and so, if you build in the same kind of headroom margin in a digital system as we've always enjoyed in the analogue world, you need to average the signal level at around -20dBFS, at least while recording (tracking) and mixing, with transient peaks kicking up to about -6dBFS occasionally.
It has become standard practice to remove the headroom margin when it is no longer required, after final post-production and mastering of the final mix, which is why commercial music averages about -12dBFS or so and peaks to 0dBFS.