Flat and Hierarchy Lookup Search Tabs 

If the search tab corresponds to a flat or hierarchy lookup field, the tab includes a list (in the case of a flat lookup field, as shown in the figure below), or a tree (in the case of a hierarchy lookup field, as shown in the figure at Taxonomy Lookup Search Tabs).

Flat and hierarchy lookup search tabs permit you to select multiple values.

When you select a node in a hierarchy lookup search tab, the entire branch (that node and all of its descendents) is selected to give the visual cue that what you are really doing is selecting leaf node values (since records can only be set to the value of leaf nodes). Moreover, the selected descendents include those that are limited out of the selection tree due to other search field selections or because no record in the repository has the final leaf value set. In this way, the selections remain constant even as the other search selections change, or as records are assigned to leaf values.