What an Origin Is
The point a direction is measured from — the question hiding underneath every position, and the one the Coord. Origin dropdown answers.
Parallax
Why the same body points in different directions from different vantage points, and why the size of the shift is decided entirely by distance.
Geocentric
Measuring from the centre of the Earth — the default setting, and the one that does not need to know where the observer is standing.
Topocentric
Measuring from where the observer actually stands — the setting that uses your latitude, longitude and height, and matters for exactly one body.
Heliocentric
Measuring from the centre of the Sun — where the Earth becomes a planet, the Sun and Moon leave the chart, and the bodies show their real orbital positions.
Retrograde Motion Is a Geocentric Effect
No planet ever reverses. Retrograde motion is what overtaking looks like from a moving platform, and it disappears the moment you step off.
Choosing an Origin
What each of the three settings is for, what switching actually changes, and why some studies will not offer you all three.