I'm not sure what they mean either. But generally you want your average signal strength to hover between -18dB and -12dB. That way your peaks, i.e. drums, applause, door slams, mics falling over, drunk guy at the bar belching, and stuff doesn't clip. Since it's all pretty much a guessing game until AFTER the event. Distance makes a huge difference in levels. And some venues are bit more live than others. Drums tend to double a groups dB output so even conservative guessing can nip you when the whole group starts playing full bore.
If you know what's going to happen, i.e. scripted, no source louder than known level, you can ride levels pretty close to 0dB. But you never really know and you can't always be lucky. Just one helicopter or 1st responder away from clipping if you don't keep a safety margin of sorts. But I'm sure that there's more accurate info out there beyond just guessing on my part. All I really know is that if I'm doing more than 20dB gain in post, I screwed up. Around 10dB of digital gain and I did alright. Less than 5dB and there wasn't something unexpected there, I got lucky at best. At worst, no do overs.