If you have a conventional joystick, i.e., it has only one handle and rotates/moves in eight directions and potentially also rotates on a Z axis (twist handle for rudder), then:
Joystick L means the joystick movements in all eight directions. X is usually roll (left or right) and Y is pitch (toward/away from you).
Joystick R is the Rudder Twist (if present on your joystick). That is the Z axis.
Notably I do not see Joystick L Z axis as a valid entry on my joystick, but that just may be a manufacturer variance.
Joystick Slider is the control for a built-in throttle on your joystick.
Sensitivity + and - just means you want to assign a movement specific to an axis. Unless that’s really what you want to do, it just makes more sense to assign the Axis ONLY to a movement - i.e., Aileron (roll) should be Joystick L Axis X, period.