Question

what is the rule for use of letters in place of numbers

Answer 1

I am not aware of any rule. Different unknowns get different letters. In physics and engineering, the letter chosen is often the initial of the quantity it represents (such as "r" for "radius"). Greek letters are often chosen for angles or continuous variables in Field Theory.

Answer 2

The exception are perhaps constants where there is a generally accepted set of symbols to replace the numbers such as pi, R, e, G, c and so on.