'Expression ON' selectors: Selecting multiple stages using one of the grouping check boxes acts as an 'OR'. This means that if a gene is expressed at or above the chosen expression level in any one or more of the selected stages it will be returned in the result list. To get 'AND' behavior (i.e., return only those genes which are expressed at the chosen level in each one of the selected stages) you must select each of the stages individually.
'Expression OFF' selectors: Selecting multiple stages using one of the grouping check boxes acts as an 'AND'. This means that for a gene to be returned in the result list, the observed level of expression must be at or below the selected level in all of the selected group stages. Therefore, for the 'expression OFF' selectors, checking a group check box is functionally identical to selecting each individual sub-category.