From the updated information I'm guessing that the BH-14 and BH-15 are Heavy Dreadnoughts, the TB-15 is a Battle Rider Tanker, and the BZ-15 is a Battle Tender
Actually, the BZ is a battleship that also carries a smaller squadron of riders. It's intended to engage in direct battle as the big gun. AFAIK, the "Z" indicates experimental.