Unconnected lookup

Informatica unconnected lookup can be taken as Oracle function. There are
many scenario's in which unconnected lookup is preferred over connected
one
1. When same lookup is to be used multiple times in a mapping
2. When conditional lookup is required. To elaborate this more ... say a
table should be looked up when column has some specific value

Hope this answers your doubts.

I have a senario ... if employee is manager than lookup his department
details. So, I dont want to do lookup for all employee types. In
expression, I will have ports as follows

in_Employee_Number
in_Employee_Type

I will create a port - v_Emp_Department = IIF(in_Employee_Type =
'MANAGER', :lkp.lkp_Employee(in_Employee_Number), '')

Now, lkp_Employee will be called only when Employee_Type is Manager. If no
manager record exists in Source data, the lookup will not be
created/called at all.

:lkp is a way of calling unconnected lookups. When u will open
transformation editor window, you will see Lookup() in functions where
to_CHAR, etc functions are listed.