Only(Field) is the same thing as Field - the difference is, that with Only() you can use set analysis.This is usually used for mapping dimensions.
Exists() is used within the script and is the equivalent of where FieldA = FieldB while FieldB is coming from a different table. This is very helpful if you want to achieve an optimized load (much faster!).
Total() does not exist. There is, however, a total qualifier within set analysis. The total indicates, e.g. in a chart, that the total of the aggregation will be displayed, irrespective of the dimensions. Without the total qualifier, the set analysis expression will evalutate for each dimension value respectively.
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