Older series display a condensed two-digit set, where the same number means different things depending on the generation showing it. Current series use a longer form — 45-1-01 — where the first pair identifies the subsystem and the rest locates it. EC codes are what the display shows when the board has logged a condition it wants looked at.
All three tell you where to look. None of them tell you which part has moved out of range, and none of them distinguish a failed component from a wiring problem or a condenser that has not been cleaned in six years. That is what the diagnostic visit is for — and it is why a code looked up online sometimes leads to a part that was never the problem.