Adjusting audio gain

http://library.creativecow.net/video-tutorials/adobepremierepro
http://www.sounderpro.com.tw/Reviw/SR.html
1. Continuing with the open project, click the Music Stereo.wav clip to select it.
2. Right-click the clip --> choose Audio Gain
There are 4 options: Set Gain to, Adjust Gain by, Normalize Max Peak to, Normalize All Peaks to. What are they?
3. Set the Normalize Max Peak to option to 1dB and click OK.
The waveform on the Timeline will expand to show the increased gain.???I didn't see it.
4. Play the Timeline and notice that the master audio meter plays between -6 and 1 dB.
5. In the Sequence menu choose Normalize Master Track
headroom:  vertical space in which to stand, sit, or move
http://audioart.audionet.com.tw/131/梁過131%20.htm
6. Set the value to -3 and click OK.
7. Play the Timeline and notice that the levels on the master audio meter have been reduced by 3 dB.