I believe that button 15 is either the pinky switch or the switch that gets activated when the primary trigger is pulled all the way back. (There are two switches on the main trigger, half-way and pulled all the way back.
I have mine set to (1) - brakes regular and (15) - brakes max.
I use (I think) button 4 to set the camera-0 view and the pinky switch to return to the camera 0 preset (straight ahead at the desired zoom level/seat height adjustment for best visibility of controls and outside view.)
If you’re not sure, or especially if you’re not sure your controller is totally OK, there are a number of web sites you can go to that perform a browser-based “gamepad test”. It will detect the gamepad, (joystick in your case), enumerate all the axes and buttons, and will show you which is which when you move/press them.
I actually made a button-assignment chart for the Saitek X-52 controller, but am not sure where it is.
I’ll post it if I find it.
Update:
Here is the site that tests the gamepad/joystick and lets you discover what is what.
https://beej.us/blog/data/javascript-gamepad/
Button 15 is the top-left hat-switch pressed upward.