aerial photography - What is the x and y camera gimbal angle for a drone?

I was looking at this question. How do I calculate the ground footprint of an aerial camera?

I'm trying to calculate the area a photo footprint captures depending on the elevation of a drone. This answer explains it very well, but I'm having trouble understanding the x and y-axis gimbal angle the answer describes.

In particular, for mapping applications a drone's camera is normally not directly pointing down, but instead point a bit forward, perhaps 10 or 15 degrees. I'm wondering if this is the x-axis or the y-axis. In most examples, both the x and the y axis angle are the same, but this doesn't make sense, since most drones, like the DJI Phantom 3 has a camera that only moves along the up and down axis.

Any help would be greatly appreciated.