Each document set has a default document subset where most documents are stored. The default document subset is represented with the Default icon so it is differentiated from the other document subsets.

For training sets, the default document subset is where all of the flagged documents for online learning are stored.

For test sets, the role of the default document subset is not as important, but still relevant. If you copy or move documents from one document set to the other, the default document subset is where these documents go if no specific subset is selected.