By default or if you hit cancel in the search field the Kanji of the current Set are displayed. If you search (by Kanji/Reading or meaning), the app searches within all Kanji of the program’s dictionary.

switch between:
Marking mode: if you click on a Kanji it gets marked for your custom selection (Settings -> Test Set -> Marked Entries)
Detail mode: if you click on a Kanji, details of this Kanji are shown in the Practice View