This clause overrides all attributes—keeping the metric from being broken down at all. As a result, the associated metric will remain aggregated at the highest possible level of aggregation across all dimensions.
ALL IN ALL OTHER DIMENSIONS can be abbreviated as "All Other."
SELECT ... BY ALL IN ALL OTHER DIMENSIONS
SELECT metric BY ALL IN ALL OTHER DIMENSIONS
SELECT Payment BY ALL IN ALL OTHER DIMENSIONS
SELECT Payment / (SELECT Payment BY ALL IN ALL OTHER DIMENSIONS)
The second example above could be useful in a report that breaks data down with a Month attribute. The above metric would show the percent share of each month's payments relative to the sum of all payments across all time and all other dimensions.