It should be noted that this is similar to the Multiverse, which contains completely separate worlds with different versions of Superman. But, for the most part, continuities are distinct from these alternate realities (though the line between them is blurry). For example, Earth-Tangent is an alternate universe with a drastically different Superman, but it's part of the Modern Age continuity - that is, the Modern Age Justice League crosses over to the alternate universe, making it part of the same continuous story.