Additions are Whistle, Finish and Clap, hitsounds that are added on top of the hitnormal that every note has. You use the dropdown to override your choice of Sampleset only for the additional but not the hitnormal.
Changing the dropdown for the sampleset always affects whether drum-hitnormal, soft-hitnormal or normal-hitnormal is used. Furthermore, if the Additions are set to "Auto" they will always use the sampleset specified in the sampleset dropdown even if the sampleset set in the timing point is different.
Some examples to make this apparent:
Timing Point: Soft
Sampleset: Auto
Additions: Drum
Additional Hitsounds: Whistle
This will result in a soft-hitnormal and drum-hitwhistle being played.
Timing Point: Drum
Sampleset: Normal
Additions: Auto
Additional Hitsounds: Clap
This will result in a normal-hitnormal and normal-hitclap being played.
Timing Point: Normal
Sampleset: Soft
Additions: Drum
Additional Hitsounds: Finish
This will result in a soft-hitnormal and drum-hitfinish being played.
All in all, you can use them to mix different hitsoundsets in one note. It is also more convenient for hitsounding in general compared to using inherited timing points.
