To be more specific....it should be called "7-way ()". But you are probably referring to a clostridial vaccine. The seven way means that it provides resistance to 7 different types of the diseases in it's category. I believe there are actually 8 known in the category and I try to find an 8 way when vaccinating, the 8th being tetanus clostridium.
Although Cl. perfringens type B is not a significant problem in North America, immunity is provided by the beta toxoid of type C and the epsilion toxoid of type D. This product consists of killed, standardized cultures of Clostridium spp., with a special, water-soluble adjuvant (Stimugen).
Here's a link to the Merck Veterinary website if you'd like to research a little more on the diseases that these vaccines are designed to prevent.