Once a filter is created it becomes associated with another entity called ‘Filter Key’. The value of this entity is the same as ‘Filter Variable’ value, selected during the filter creation process.
‘Filter Key’ is basically variable, that can take values depends on what the user selects as filter values.
‘Filter Key’ itself can not filter the data on the report. In order to achieve it, the report dataset should have declared parameters, and the dataset itself should have all logic, to return proper data based on these parameters values. So, ‘Filter Key’ will only pass values from the filter to the dataset parameters.