Your middle score is typically used by mortgage lenders when determining your mortgage eligibility. To obtain your middle score, your lender will obtain a credit score from each of the three major credit bureaus (Experian, TransUnion, and Equifax). Of those three scores, your middle score is the second-highest (middle) score.
A quick call to your potential lender can help you uncover which specific scoring model they will request from each of the three major credit bureaus.