What's meaning of formula should used?

Today I saw the question Denoting the set of initial segments of a binary sequence, which searches for a formula. It got put on hold as an unclear question. I guess that the unclearness was from meaning of formula. So can any one explain applicable meaning for it?

• Sorry, I couldn't make sense of your title, so I left it untouched. Oct 30 '13 at 15:41