Key signatures with up to 6 sharps or 6 flats may be selected. Each key signature is used for two keys, the Major key and its relative minor key. For example, G Major and e minor use the key signature of one sharp.
In MusicGoals the key signature of C Major / a minor has two possibilities. One displays accidentals as flats, and the other displays them as sharps. If you play F# or Gb, the staff notation will be determined by this choice. However, for the intervals and chords, the required accidental will be used. For example, if you select C Major / a minor-use flats, a Major second above E will still be shown as F# in the staff notation.
see also: Reading Key Signatures