If you dig deep you discover they have figured out some requirements in detail, but there is a lot missing. Is this new feature that last one in that line, or will there be more options in the future? Is this new feature really going to be used - many times we have put large effort into features only to discover no customer used them (as evidenced by the critical bug that made the feature unusable outside of the test lab that nobody complained about until 4 years had passed). These things drive how you engineer the thing in the first place.