It will disable the starter interrupt feature if pressed once - not relevant to this installation as I did not add the starter interrupt - needs a further relay. Valets often leave cars unlocked, and if the brain times out in this state it will re-enable the starter interrupt, prohibiting the valet from starting your car with the key.
Now your car, by contrast, may be worth stealing, so you may want that feature.
Practically, the button is used to program the brain as set forth in the last slide. There's a lot of functionality. I just illustrated the major features. One to think about is 'relock doors after open, in 90 seconds'. You drive down your driveway (doors locked when you started) and open the driver's door to get the paper. All doors unlock and stay unlocked. They will relock in 90 sec if you enable this feature. Trouble is, what if you step out and leave the car running with the driver's door closed? You lock yourself out. The ideal solution is a sensor hooked into the car's VSS, but I have no idea how to do that. I believe the W210 and W220 use this approach - not sure. A simpler approach might be to hook the brain up via the driver's seat belt and only allowing door re-locking if the seat belt is fastened. I think I could work that one out.
As yours is a later car (sorry, I'm clueless about the W124) it may have a vac actuated driver's door lock - check by seeing if you can lock the driver's door using the trunk lock. If you can, it does.
If that's the case, consider perusing the instructions for the 'late W126' on my site. Much, much simpler, as you do not have to pull the driver's door panel and install an auxiliary actuator. Your pump may be under the rear seat rather than in the trunk (as on the W126), but the wiring s/b similar - three wires to the pump, a ground and two others (lock/unlock). You splice into one of these others with the relay setup illustrated.