It doesn’t really matter since we are just talking about notation. You could use stick figures to represent your variables and it would all still work the same.
But conventionally you see a lot of Greek letters, often with discipline-specific meanings.
Ultimately if you need an inexhaustible pool of variables to choose from you can just use letters with subscript numbers (X1, X2 etc. except those numbers should be subscripted and i can’t do that in reddit)
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