Generating unique, unambiguous, IDs for data is something we do often, but we do not always know what level of uniqueness is really needed. In some cases, we want to be really sure that two instances of the same ID identify two copies of the same object or data. In other cases, we only want to be reasonably sure. In other cases, yet, we just assume that collisions—two different objects yielding the same ID—are very unlikely, and, if the need be, we can proceed to further testing to establish equality.
There are many ways of generating IDs, each with varying levels of confidence on uniqueness and differing applications.