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What is important is to evaluate the magnitude of a typical action (the physics action) and compares it to the Planck constant. If the latter is negligible, a classical description would work.

Thanks for the answer :) I am going to look more deeply on the Planck constant and understand the consequences.

Apart from being the smallest quantity/measurement and using it in equations during college, I know nothing about it :)

it is used a lot in the equations behind many applications used in real life (I mean, beyond textbooks). But no one tells it ;)

I will read on this and get back to you in case of further questions :)