1. Calling the same lookup multiple times in one mapping 2. Testing the results of a lookup in an expression 3. Filtering rows based on the lookup results 4. Marking rows for update based on the result of a lookup,such as updating slowly changing dimension tables
If You want to use the same look up Target more than one time,you should use compulsory Unconnected lookup.A common use for unconnected Lookup transformations is to update slowly changing dimension tables.
performance wise unconnected lookup is better because its like reusable one and it will lookup the values only the condition meet the criteria.
1. It will return only one value 2. It will act as static cache