With relative throttle you can set your speed between fixed values, which are exactly Zero, 1/3, 2/3 and Full Speed of the ships' maximum velocity iirc.
With absolute throttle you can set your throttle to how you like it. So to speak: You have absolute control over the throttle gauge 😁
That's not quite right. Both options let you manually set your throttle to whatever you want, but they're meant for different control devices. An absolute throttle axis has defined physical minimum and maximum input values, like a joystick's throttle slider or the actual throttle of a HOTAS. Your throttle input is defined by how far your control is set above or below those physical stops: if you want your ship's speed set to 40% of its maximum, then you move the throttle axis to the 40% position and leave it there. In contrast, a relative throttle axis sets your throttle up or down compared to its current position. It would make sense when used on something like the wheel on a mouse, or maybe a thumbstick axis on a controller, cases where you either don't have physical stops on either end of the axis, or it's impossible to have that control stay there. In this case, if you want to set your speed to 40% of maximum, you scroll the axis up or down until you hit that number, then stop using it.
(I think you were thinking of the keyboard commands that let you set those fractions of the throttle.)
