In the context of a Let statement the parser knows you are refering to variables. This is why Let Y = 7 + X; works without expansion. In the context of load statements it will interpret Y without the expansion as a field name (i.e. if you had a field in your dataset that was called "Y" you'd want to reference that - not the variable). Expansion is the way of telling the parser to leave the contect it's currently in to evaluate the statement differently (in this case look for a variable name).