If you select the display preference option Allow Search When Number of Pages Exceeds __ and the number of members on the open form exceeds the number you set, then the Search icon is enabled. (See Enabling Search with a Large Number of Pages.)
You can search by member name or alias. If the selected member in the drop-down list is the first member of the hierarchy and you search up, the search starts from the last member of the hierarchy. Similarly, if the selected member is the last member in the hierarchy, the search starts with the first member.