Think of deferred coordination as "deferred population." That is, when a coordination-causing event occurs in the master block, the population phase of coordination is postponed, but Oracle Forms clears existing records in the detail block immediately. This functionality prevents a detail block from displaying records that are inconsistent with the current record in the master block.
As a general rule, choose immediate coordination (Deferred = No) when the detail block is visible to the end user, and when it is assumed that the end user will always want to see the detail records associated with the current master record.
Deferred coordination with Auto Query is preferable when the detail block is not immediately visible (to view detail records, end users must navigate to the detail block in a different window).
Deferred coordination also is useful when end users may not need to view the detail records at all. In this case, deferring coordination can prevent an unnecessary database query.
Deferred coordination without Auto Query is useful when you want end users to be able to switch to Enter-Query mode and specify additional query criteria in the detail block before population occurs. This also allows end users to navigate through detail records without forcing coordination, and its attendant processing, to occur until it is actually required.