Learn the algorithm--num operation in MIX from the beginning

You've just seen the mix in the textbook order from the trip.
Num is a conversion operator that converts character encodings to numbers.
It converts the value of Registera & Registerx to a number and assigns a value to Registera.
The conversion process is byte-wise:
00,10,20,30,40,... Converts the character to the number 0;
01,11,21,31,41,... Converts the character to the number 1;
02,12,22,32,42,... Characters into the number 2, and so on.
If there is an overflow, Registera = value MoD b^5,b is the size of a byte.

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The question is, what is the mathematical significance of this operator? Why is there such an operation symbol?

Learn the algorithm--num operation in MIX from the beginning