The Seventh Astrological House - Setting Place
The Seventh House: Also known as the setting place.  Paulus says that this place gives signs for quality of death, wedding preparations, long terms abroad and the elderly age.  Modern astrologers often assign this place to partnerships and weddings but not to conditions in the golden years or to quality of death, but the latter significations are common in the older texts. This signification concerning the golden years of old age and quality of death comes from the idea of primary direction (the directing of the planets from sunrise to sunset according to the earths rotation; also known as the "circle of the Same"), where sunrise represents youth, the midheaven; middle age, and sunset; the golden years. There are also possible religious connections for this such as Zoroastrianism and Judiasm where the setting Sun was viewed as symbolic of death. The reason why this place is considered an indicator for long stays abroad is not explicitly known as far as I know. However, I do have a guess. This place is the farthest that you can get from yourself in the zodiacal circle. If there is an astrological indication that you will become familiar with this place, then it will naturally take a long time to come back to home to yourself both physically and metaphorically.
Dane Rudhyar said that the ascendant/descendant axis pertains to consciousness and awareness.  It is at this setting point that the awareness turns to the other; part of which is known (that which is above the horizon) and part of which is unknown (the portion of that sign that is below the horizon). It is of course true that there are things both known and unknown about any partner or adversary. (This is another reason to take the whole sign that straddles the setting point as representing the affairs of the seventh house, and for using the whole sign house system for topical issues.) The more degrees that passed above the horizon for your rising sign at birth, the more visible, known and vulnerable you become. The opposite must also be true, because if your ascendant is mostly visible the descendant is mostly invisible and therefore secretive to you. There is a power component here related to altitude even in the ascendant/descendant axis. Dane Rudhayr considered the 4th - 10th to be the main power/authority axis (of which I agree). 
In medieval texts, especially concerning horary astrology, this place was considered an indicator of war and open enemies. This is because of the confrontational nature of this place; where we come face to face with our opponent. If this confrontation is resolved amicably, it becomes friendship, partnership and even marriage. Psychological astrologers often refer to the 7th house as the place of projection, and some even call it our shadow. I tend to think of the 12 house as representing shadow characteristics because we don't often come face to face with our shadow; which I think is better represented by the astrological concept of aversion.
More info on each of the planets that have dealings with this place:
19. Ibid. (See note 14).
21. Dane Rudhyar. The Astrological Houses. © 1972 Library of Congress Catalog card Number 74-180105, ISBN 0-916360-24-5 Originally published: Garden City, NY: Doubleday 1972.
23. Paulus Alexandrinus, Hellenistic astrology Introductory Matters 378 A.D. © 1993 Robert Schmidt, through Project Hindsight, Published by The Golden Hind Press. See pg. 53.
24. Ibid. See pg. 54.
26. Ibid. See pg. 55.