<field> is a syntax used with the total keyword. Ususally, total means that the aggregation is done over all values regardless of dimension. However, it is possible to make it take some dimensions into consideration. So if u have dimensions year and region the expression: count(total <region> sales) will count the total number of sales for each region, regardless of year. Count(total <year> sales) will count the total number of sales for eac year, regardless of region.
Hope it helps!