TransUnion is one of the three major credit reporting agencies, so a large number of lenders may pull your TransUnion credit report and associated score. With most lenders, you will not be told who they pull their information from until you are approved/denied for a loan or you do not receive the best interest rate possible.
TransUnion uses a scoring system called EMPIRICA. This system has a much wider range of scores than the more traditional FICO scoring system. The EMPIRICA system ranges from 150 to 934. Your EMPIRICA score is usually higher than your FICO auto-enhanced score, unless you have been reckless with your past use of auto loans.
Some lenders will look at your TransUnion score if your credit score from FICO is somewhat marginal. The higher EMPIRICA score may be used in order to get your loan approved.
The large banks that have been known to use TransUnion are Chase, Citi, Discover, and Wells Fargo. That is a short list of national banks, and many local lenders may use the score without it being widely known.