Answer: This kind of sounds like a quote from an exam page, though I expect the exam is referring to 'accidentals.' That's an easy mistake to make. But by now you're out of the exam room; your fate is sealed one way or another, and I feel free to talk.

To get started: somewhere in Exploring Theory I say that these marks may be called accidentals but they're actually done on purpose. In fact I may have said that more than once; I like it.

An accidental is any sharp, flat, natural sign, etc. that appears beside a note in the course of the music. That is to say, a sharp or flat that is part of the key signature is not an accidental, the word only applies when you see the mark beside a note.

The sharps or flats in the key signature provide whatever is needed for the current scale. For example, if you're playing in the G major scale (in the key of G major) then you need to sharp the note F whenever it appears. So that F sharp is placed in the key signature and you don't need to bother writing it in each time.

Since the key signature provides what is needed for the key, the first time you see an accidental in the course of the music you know that here is a note that is not in the current key signature. If it's a major key, using the ordinary major scale, then this means you're borrowing a note from another scale.

But if the music is in a minor key there are some accidentals that appear all the time. The "natural minor" doesn't have any; it's like the major scale. But the harmonic minor raises the 7th note of the scale, using either a sharp or a natural sign, as appropriate. The "melodic minor" raises both the 6th and 7th degrees when the melody is going upward, leaves them natural when going downward.

Short answer: If you see some music that has no accidentals in it (the key signature doesn't count) then it is all in the major scale, or all in the natural minor scale, or all in one of the "church modes" like Dorian, Phrygian, Lydian, or Mixolydian.

If you see any accidentals, then either it's a minor piece with some raised 7ths and/or 6ths, or it's in either major or minor but also borrowing some notes from other scales - it might even be modulating to different keys.