As you can see in the Reference Manual, they are used to assign values to variables in the load script. The main difference is that while SET assigns the value to the variable without evaluation, as a string, the LET first evaluates the expression at the right part of the "=" sign, then stores the value.
Two simple examples:
SET vLiteral = 1 + 1; // This stores the string "1 + 1"; LET vSum = 1 + 1; // This stores 2
Hope that helps.
Let and Set commands are used to define Script Variables. Any text or numeric value can be assigned to the script variables.
Set V = $ ; ( Set- assigns the variable(V) the text(String) to the right of the equal sign)
Let vStart = num(makedate(2011)); (Let- evaluates the expression)
Set MUL = ’$1*$2’;
Set X = $(MUL(3,7)); // returns '3*7' in X
Let X = $(MUL(3,7)); // returns 21 in X