From the Search area in the Circuit form tab:
The search (pull-down) field is used to search for Circuit names matching the entered string in the field ... or ... when clicking the pull-down icon and using the search fields at the top of each field column to filter your search
The Search Text field is used to search for sites whose information includes what is contained in the Search Text field i.e., typing in 2MB would return records where any field of information contains 2MB, say, in the Name or Speed columns.