Point of launch which is typically from where your remote is unless you're taking off from a launch pad else where. Or you're not flying VLOS. Pretty sure those commenting just want to troll and know exactly what I was saying.
I worry more that people might read your original post and
not understand exactly what your were saying, or worse read it and take what you wrote to be
literally true, even after you edited your original wording.
It uses the distance from your point of launch [edit: which is typically darn close to your controller] to calculate AGL.
It's not distance, it's height.
Your "point of launch", if from the ground, can be several feet
lower than your handheld controller, if you're 6'-3" tall like me. And the
distance from "your point of launch" can vary by a lot if you launch from a moving boat, or even if you just walk any distance while flying your drone.
The sensor is not even actually measuring height. The sensor measures barometric pressure, with changes in that pressure converted by the flight controller to display as a height reading.
Zero height is registered at the moment the motors are first started (at least it is on my Mini), which also is not necessarily the "point of launch" because you can start the motors while the drone is sitting in your hand and carry it to a different location before setting it down to launch. I made a video demonstrating this just recently, showing that zero height is recorded when the motors are started.
After that, the displayed height is in reference to that recorded zero point, regardless of how high above ground (AGL) the drone actually is flying over varying terrain elevations.
Accusing others of not being able to read, or telling them to read their user manual, or take a Part 107 course, in order to understand what you wrote isn't going to win you many friends.