The "Single package" rule assumes you will pack all your products ordered into a single package for mailing. The dimensions that are passed to the shipping provider will be the external dimension of the package itself regardless of what you actually store within it (i.e. it does not validate if products will actually fit since you decided this is the package to use). However, the total weight submitted to the shipping provider is the sum of the weight of all your products plus the weight of the empty package.