Final answer:
Boolean subexpressions ei and ej are independent if they do not share any variables, allowing their evaluations to occur separately with no influence on each other.
Step-by-step explanation:
Boolean subexpressions ei and ej are independent if they do not share any variables. This means that in the context of Boolean algebra, the evaluation of one subexpression has no effect on the evaluation of the other. For instance, if ei = (x AND y) and ej = (w OR z), then ei and ej are independent because the variables x, y are different from w, z. However, if ei = (x AND y) and ej = (x OR z), they are not independent since they both involve the variable x.