When a recipe calls for a "15 oz. can of diced tomatoes," it refers to the weight of the entire can, including the tomatoes and the liquid in the can. In this case, "15 oz." means a can that holds approximately 15 ounces (425 grams) of diced tomatoes and their juice or sauce.
When using canned ingredients in recipes, it's essential to include both the solids (tomatoes, in this case) and the liquid from the can unless the recipe specifically instructs you to drain the liquid. The liquid in the can often contains flavor and nutrients that contribute to the overall taste of the dish. If the recipe requires draining the liquid, it will usually specify that as well.