As I understand it, for fighters the enemy aircraft had to be seen to be destroyed (i.e. blow up, break up, hit the ground) or the pilot be seen to bail out, either on cameras or confirmed by a third party to be confirmed. If your claim was on fire and diving vertically at the ground, but you didn’t see it go in, it can only be counted as a probable.

For bombers, whoever claimed it was awarded it, leading to massive over claiming.