A fruitful one signifies spiritual good from charity.

(références: Apocalypse Explained 357)

1529. It is very well known in heaven, but not so well known in the world of spirits, where so great a light comes from, namely from the Lord. And what is astonishing, the Lord appears to celestial angels in the third heaven as the Sun, but to spiritual angels as the Moon. There is no other source of light. But the intensity of the light angels experience depends on how celestial and spiritual they are, and the nature of the light depends on the nature of these qualities. Thus the Lord's celestial and spiritual itself shows itself before angels' external vision by means of the light.