When working with an algebraic formula it is often necessary to manipulate a portion of the formula rather than the formula as a whole. Calc allows you to "select" a portion of any formula on the stack. Commands which would normally operate on that stack entry will now operate only on the sub-formula, leaving the surrounding part of the stack entry alone.
One common non-algebraic use for selection involves vectors. To work on one element of a vector in-place, simply select that element as a "sub-formula" of the vector.