Excerpt from B. Dykes' "Using Medieval Astrology"
"When a planet is in aspect to the lord of its domicile, or exaltation, or of at least two of the other dignities, it is 'received', by that lord, and the lord is the 'receiver'. For example, let Venus be in 25 Sagittarius and bear a trine aspect to Jupiter in Leo. Since Jupiter rules Sagittarius by domicile, he receives her there, and she is received by him. Put simply, they are in 'reception'. Or, suppose Mercury is in 25 Pisces in a nocturnal chart, and he trines Mars in Scorpio. Mars is neither the domicile nor exalted ruler of Pisces. But he is the primary triplicity ruler by night, and he rules Mercury's term. Since he rules two of Mercury's minor dignities in that position, he can receive Mercury.
Another view is found in William Lilly, and expounded today by John Frawley. Frawley and Lilly claim that reception takes place simply by being in the dignity of any planet whatsoever, - i.e. that no aspects are needed, and that a planet can be received by its triplicity, or term, or decan ruler alone. So if any planet is in the first decan of Aries, it will be 'received' by Mars, regardless of Mars's condition or any aspect between them. Now, this definition is really what medieval authorities called 'generosity', a very minor situation that holds between any planet and whatever planets rule its degrees at any time whatsoever. To me the difference between generosity and true reception proper is the difference between someone's being in your home (generosity) and actually greeting them while they are there (reception). If you go to someone's house and the door is unlocked, but nobody is home, you are not exactly being received. Aspects confer that kind of greeting, recognition, perfection, reception."
My personal comments:
1. Although it is considered to be a minor situation, 'generosity' is still a good indication of 'who likes whom or what' in horary.
2. Receptions are crucial for the perfection of the matter in question. For example, trine without reception will most likely signify missed opportunity, opportunity that presented itself easily but it was rejected in some way.
3. From my experience, Lilly and Frawley got it 'in reverse', i.e. Venus in Capricorn making aspect to Saturn or Mars will be well received because Saturn 'likes' everything coming from the place of his domicile and Mars 'likes' everything coming from the place of his exaltation, but Venus in Capricorn making aspect to Jupiter won't be received because Jupiter 'doesn't like' everything coming from the place of his fall. So it's "Saturn loves Venus" and not Frawley/Lilly "Venus loves Saturn".