Hello Narenda,
in addition to Martin's reply:
In many transactions the user starts without knowing a technical BOPF key at all, but only knows some certain alternative key values (some kind of ID, e.g. sales order id) or a certain attribute value (amount > 10.000 EUR) of the instances to be processed. However, to consume BOPF services like for instance modify, retrieve or action execution, the consumer requires the technical BOPF key (as this is the IT_KEY importing parameter of those services). To get those technical keys either a query or an alternative key can be used.
Thus use a query for selecting node instances to get their technical keys required for further processing. If you use the QUERY core service on the service manager and specify parameter IV_FILL_DATA = X, then BOPF does first execute the query and afterwards the RETRIEVE for your convenience.
Hint: The RETRIEVE core service also is able to access the database if you use it on service manager level with parameter IV_BEFORE_IMAGE = X.
Best regards
Tilmann