A planet in another planet’s domicile or exaltation will be seen as an ally by the ruling planet. If it’s in another planet’s fall it’ll be seen as an enemy by that planet. If it’s in the exile/detriment of another planet it’ll be seen as an “alien” or an “other” by that planet.

For example if the Moon is in Aries it will be seen as an ally by Mars and the Sun, as an “other” by Venus and as an enemy by Saturn. So the Moon’s aspects to the Sun and Mars will be more effective, and its aspects to Saturn and Venus less effective.

The relationship between planets is not just described by aspects; it’s also described by these sign placement factors, which add another layer of data or information for the astrologer to integrate into the reading.