I have found the answer.
A private API refers to any undocumented methodology in Apple's own API. Apple restricts this because if your application relies on calling an undocumented method in the main Apple structure, and then this undocumented method changes or completely collapses in the next OS update, your application will break and is useless by itself.
Using external APIs, such as the NinevehGL engine. Using Im is excellent, assuming that NinevehGL does not use any private APIs.
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