think of service menu a much more complex adjustment/calibration than standard On screen Menu aka. GUI.
You know on screen Menu,
It usually have this:
Picture or contrast
and sometimes color temperature
and depends on models/brand SVM, aka VM aka VSM and other marketing names...
But service Menu is much more complext:
it let you adjust the color decoder to be accurate such as from red push, green push, blue push to accurate color.
And color temperature to be accurate close to D6,500k which have a true 100% white and greyscale...
these two are very important on how the color looks on your tv set. Beside that, if your tv doesn't allow you to turn off the VSM, SVM, VM, etc. on the on screen menu. You usually can on the service menu.
And what's not, it let you adjust the geometry, overscan, and even more on the audio speakers adjustment that you can't perform on basic on screen menu.
But the biggest reason why its called Service Menu and requires some sort of combination of numbers on your remote control other tv or both is because if you don't know what you are doing, you can killed the tv.
That's why Service Menu is not easily accessible on your remote.
For beginner, buying the SoundandVision Tune Up Disc, DVE, or AVIa can guide you on how to adjust basic settings.