In my other articles I often mention that planet this or that in their ruling sign have certain special properties. Here I will explain what that means.
Each planet can be anywhere in your natal chart, but not every placement is equal, some placements are better than others. There are 4 special cases of placements: domicile (rulership), exaltation, detriment and fall. Different planets can reach these states in different signs. I will describe what each of these states means for a planet.
Domicile (Rulership). You often hear from astrologers that say Mars rules Aries or Sun rules Leo. While this is not untrue per se, the wording is tad misleading. It does not mean Sun rules over all Leos or Mars over all Aries. What it means is that this or that planet functions best in this or that signs. Planet feels at home in this sign.
Every planet has one or two signs that are best align with it function and purpose. When planet is in its ruling or home (domicile) sign in a role it was born to do, it can function at its true full potential. A planet in its home sign is master of the house, has home advantage and can function completely unrestrained with its full power.
In contrast any other placements in sings ruled by other planets will limit any given planet ability and force it to adapt to an environment different from its preferred one.
A person who has any planet in domicile, can be certain this planet and abilities it governs will perform well. This is a talent they can use to get ahead in life. Of course, some discretion is advised as just the fact that planet works in its full power does not mean the abilities it governs are needed in any given moment.
You can use chart above under ruler column to tell what sign in home to which planet.
Detriment is the most unfortunate placement for a planet. For each planet detriment is in the sign, polar opposite of its domicile. In detriment a planter is clearly outside its comfort zone and unable to function properly. Like elephant in a China show, no matter what it does, it causes trouble.
Planet in detriment may have to assume a role, very uncharacteristic to it. Mars in Libra does not jump into action but plots and bides time. Venus in Scorpio dresses to avoid attracting attention.
If a person has any planet in detriment, they would be well advised to exercise caution in relying on abilities government by this planet as they can cause them more problems than give benefit. The planet in detriment does not work as its usual self and its futile to expect it of it. Doing so is surefire way to shoot oneself in a foot, it's called detriment for reason. Even if detriment placement can amount to anything it is likely to be very unlike the planet's normal role in the chart.
There are two other placements of special significance Exaltation and Fall.
Exaltation is a lesser dignity compared to domicile. You can compare it to being guest of honor of sorts. Planet is still limited by the purpose and power of the sign it is in, but in much less restrictive manner as the sign gives it enough room shine and be visible, hence term exaltation (praise).
Fall is when planet is not very visible in its effects. In some cases, it can work behind the scenes and even pull strings from the shadows. Unlike detriment however, planet is fully functional, just that its functions are obscured by something.
Finally, there are all other placements that do not fall in any of the above categories. These placements are niche, they are outside of planet's comfort zone, but still allows it to do its thing, even if in a field that is not its own.
These placements are all situation but could be of great utility to the native if certain conditions are met.
With this information about special placements of each planet one can know how each of these dignities or debilitations can affect them or people they know.
A little final word. Rulership of signs are arranged by their distance from the Leo and Cancer. Closest to Sun Mercury rules Gemini and Virgo, furthest from Sun Saturn rules Aquarius and Capricorn. The rest are in between. The picture below.