What kind of black technology is "APO" in Leica camera lens?

It's a very old technology, and there was an APO lens decades ago.

APO is apochromatic. Generally, lenses should be corrected for chromatic aberration. After correcting the first chromatic aberration, it will inevitably lead to the second-level error. Most lenses are corrected for the first chromatic aberration, because it is enough for daily use shooting, while APO needs to be corrected for several levels, which means that each lens in each APO lens needs to be accurately matched, and even the rotation angle of the lens is precisely adjusted. Often, there are two same APO lenses. Therefore, once the lens of APO is dismantled, it is meaningless, and it must be proofread again at the original factory. To put it bluntly, it is to get married and buy jewelry. 99.99 thousand gold is enough, but some industrial gold raw materials may need 99.999 purity. Every decimal point means that the cost is several times higher. This is why APO lenses are expensive.