What do "Unamplified" and "100%" mean when describing "Output volume"?

You are seeing this because your sound card supports amplification beyond the normal range of the actual volume. Unamplified is the normal maximum, the next marker (100%) represents the maximum amplification that your sound card supports (ie how much your sound card can amplify sound on it's own), and the highest one (the unmarked end of the slider) is the amplification that is available from pulseaudio (on top of what your sound card supports).

The safest is Unamplified%, but 100% (the second marker) might also be safe depending on the volume of what you are playing and your speaker quality. If you are playing very low volume audio files, and you don't want to use normalization, you can use these features to your advantage.