Replay Gain Adjustment
The number you see next to "Replay Gain (for current track)" in DSP Studio is the adjustment value calculated by the analyze process. When Replay Gain is enabled in DSP Studio, the program uses this value to adjust the track volume when playing it back. Pretty much all tracks will playback softer than they would without replay gain. You can play the track using this value, or you adjust it by manually entering a number, or letting the program do it automatically.
When this is selected, the file will playback with the "Replay Gain" for the current analyzed track. If that track is not analyzed, it will playback at the average of all the analyzed files currently being played.
You would manually enter an adjustment, so that files are always played back at their calculated Gain levels + whatever number you include here. See the example below.
Automatic based on current playlist
Plays the playlist as loud as possible without clipping. (but still at the same volume). For example, if you play a loud rock album, replay gain turns down the volume for the entire album. Automatic based on current playlist will play all files as loud as possible, and only turns down the volume for the files that are actually louder than the others in the playlist. In other words, each playlist is as loud as possible while being the same volume *inside* the playlist. The volume *between* playlists will be very different.