There are two types of lookup windows used in the Media Library Manager. The first example is looking up to a related file that the user MUST select from a list. These lookups are commonly used when entering data. For example when entering the author.
Lookup example
The other type of lookup is used by the Query Generator. Here the only option is to select from the list. The user can not add to the list. A query is to find records that match the criteria. If the user were allowed to add the query would fail because there can not be any matches to a new entry.