All search criteria are independent of each other. You may search on one or more criteria.
If multiple criteria are selected, they restrict the results further. Results must satisfy ALL selected criteria to show up.
If you are not getting any results, it may mean that you have selected a combination of search criteria that do not exist together. Try searching on fewer criteria, or try searching with the default ("All") in all fields.
Example: if you select a school name, and you also select a county that DOES NOT MATCH that school, no results will show up. You must either put the correct county for the school, or remove the county and search on "All" counties.
Notes regarding make-up sessions:
"Unit" without "Month": The search will find any occurrence of the selected unit that has not occurred yet. (Taking into account time as well as day.) So, at noon today it will find any occurrences that will begin this evening, tomorrow, next week or next month; but it will not show any occurrence that began this morning.
"Unit" and "Month" together: The search will find any occurrence of the selected unit in the selected month.
"Month" without "Unit": The search will find those classes whose date range (first unit of the entire series) BEGINS in the selected Month.