Ah the mystery of watts (power) versus dB (what we hear)...
Amps and receivers produce and are sold based on power (typically at 8 ohms or maybe 4 ohms). Speaker efficiency is based on XX dB/w/m, which means so loud at 1 watt of input 1 meter away. So put together what does this mean? First you should find a sound pressure meter (spl, check your smart phone for an app) to learn what different spl's sound like. If like most you'l find you comfortably listen to around 80 dB. But music is made up of peaks, so the dynamic range can vary by 10 dB for rock, 20 dB for jazz, and 30 dB for classical.
Add in two confusion factors:
1.) How many speakers will you be listening to? How big of room will you be listening in (and how far away from the speakers will you be). Doubling the number of speakers adds 3 dB but moving twice as far away cost 6 dB for most speaker designs.
2.) The relationship between power and sound is logarithmic, meaning it takes ten times the power to sound twice as loud (an increase of 10 dB). So while most speakers are rated at roughly 85 dB/w/m and in-room you might use 1 watt/channel to reach 82 dB in-room, those peaks can take up to 500 times the watts to reach live concert levels. So speaker efficiency can be more important than wattage. Also note that pushing the amp beyond it's rated power (called clipping because on a scope the waveform flattens out at the top and bottom). This clipping is extremely hard on speakers and can cause damage quicker than applying too much clean (unclipped) power. This issue is somewhat overstated as live music peaks (105-110 dB) are quite loud and few routinely listen that loud, but accidents do happen. OTOH note that for best sound the amp should have a "commanding grip" on the speaker so all around it's best to oversize the amp for the given speaker/room application.
Getting back around to how wattage levels are displayed, professionals use dB's (like an old fashioned VU meter) to display how close to clipping they are. In this case it appears that the manufacturer is equating 0 dB of gain to 1 watt (the rated speaker efficiency). So +18 dB is about 65 watts per channel. IMO the best system (not yet invented) would be to dial the amp to the speaker/room combination so it displays an approximation of the actual dB you're listening to. Different manufacturers calibrate their displays to different arbitrary settings.
And since we're in the HT circle I'll point out that nearly all multi-channel receivers do not put out the rated wattage simultaneously into all channels due to the fact that they share one power supply. OTOH rear/ATMOS channels don't need the power of left, right, and center channels.
