All planets (except for those in their home sign) are dispositing planets; that is, they are reliant or dependant on other planets in the chart(including other people's charts) for self-completion or fulfillment. Disposition works through rulership. Sun in Aries, for example is disposited through Mars, which may in turn be disposited through another planet. It is possible to have a chain of dispositing planets such as the following:
Sun in Aries/Mars in Gemini/Mercury in Taurus/Venus in Aquarius/Uranus in Sagittarius/Jupiter in Leo/Sun in Aries. In such a case, all of these planets work together in some way.
A planet in its own sign stands alone and can freely give or refuse to give of itself to the rest of the chart. Even the transits and progressions of other planets to it don't have much effect. A planet in it's own sign is a MASTER planet and needs no one or nothing (except other master planets). Dispositing planets, on the other hand, reflect all of our needs and interdependencies.
Mutual Reception
Two planets in mutual reception are the next best thing to a planet in its home sign, especially if they are of the same element or polarity. Two planets are in mutual reception when they disposit each other.
Example 1: Sun in Aries and Mars in Leo
Each planet is in the sign the other rules so that there is a cross-link or interdependency. Both planets complete or fulfill each other, and in a positive way, since both are trine aspect (by sign).
Example 2: Venus in Aquarius and Uranus in Taurus
These two planets will also fulfill or complete each other but it won't be easy as the two are in square (by sign).
Mutual reception can also work by transit or between planets in two people's charts. If you are a Sun in Aries, for example, you will be especially sensitive to the transit of Mars in Leo, or to someone's Mars in Leo.