Philips (or any other manufacturer) made their CD-i players with three buttons in mind, 1st, 2nd and 3rd, and a directional pad or mouse. So what your adapter does is mimic all these functions, or one might say, translate SEGA controllers input to language that CD-I understands.
It is quite simple as you might guess, button A corresponds to 1st button on CD-I controller, button B is for 2nd button and button C is for 3rd. Now, the CD-i controller does not have Start button, so the Start button on SEGA controller mimics C button. From my experience, most CD-i games and applications require 3rd button to be pressed when starting or pausing, so Start button is most intuitive. Directional pad works as usual directional pad on CD-i controller.
Find the instructions here
Buy it here
[Thanks, Devin, Lauraiss’ gizmos]