The Total function deserves its own category because it functions a little differently from the others. Unlike the other functions that work on the aggregate table in the cache, Total will re-query the underlying source for all the source data rows that make up a given partition. In most cases, this will yield the same result as a window function.
For example, Total(SUM([Sales])) gives the same result as Window_Sum(SUM([Sales])), but Total(AVG([Sales])) will possibly give a different result from Window_AVG(SUM([Sales])) because Total is giving you the actual average of underlying rows, while the Window function is averaging the sums.