I think for the purposes of this standard, the word “complicated” is relative; that is, an expression is complicated if it has parts that can be regarded as a single entity (or not). For example (x+y)^2 could be viewed as complicated because it can be viewed as something squared; the key ability here is the ability to clump the x+y into a single something in your mind and see the expression that way. It might have been clearer to use the word “compound” rather than “complicated”.