What does the classifier parameter mean in mikrotik (in the queue tree)?

Question

network hardware

If more precisely, they could not translate the idiot into a simpler language, what is written here https://wiki.mikrotik.com/wiki/Manual:Queues_-_PCQ
In the middle.

I built a queue tree with traffic prioritization, but in the queue type selection line I simply indicated:
psq-download-default there is also psq-upload-default
Is this a mistake? I marked the traffic in both directions and only allowed everything through psq-download-default .
Does this break something or will it not work correctly?
From the Mikrotik manual, I thought it looked like optimization. Although twisting in my head how the processor works, I understand little what kind of threads it means.
Threads of processor cores or some other.